•AMERICAN 
TELEPHONE    PRACTICE 


BY 

KEMPSTER   B.   MILLER,   M.  E. 


SECOND  EDITION. 


NEW  YORK 
AMERICAN    ELECTRICIAN   COMPANY 

^120  LIBERTY  STREET-- 


TABLE  OF  CONTENTS. 


CHAPTER  I. 

PAGE 

HISTORY  AND  PRINCIPLES  OF  THE  MAGNETO  TELEPHONE,  i 

Early  Knowledge  of  Electromagnetism — Work  of  Oersted,  Ampere, 
Arago  and  Davy,  Sturgeon,  Faraday,  and  Henry— Transformation 
of  Eleptric  into  Magnetic  Energy— Transformation  of  Magnetic 
into  Electric  Energy — Field  of  Force — Morse's  Telegraph — Reis: 
Telephone — Sound  Waves— Bell's  Telephone— House's  Electro 
Phonetic  Telegraph. 

CHAPTER  II. 
HISTORY  AND  PRINCIPLES  OF  THE  BATTERY  TRANSMITTER,  10 

Gray's  Variable-Resistance  Transmitter — Berliner's  Transmitter — Elec- 
trodes in  Constant  Contact — Carbon  Electrodes — Demonstration  of 
Advantages  of  Loose  Contact  by  Hughes — Hughes'  Microphone — 
Running's  Granular  Carbon  Transmitter— Induction  Coil  with 
Transmitter— Advantages  of  the  Local  Circuit. 

CHAPTER  III. 
THE  TELEPHONE  RECEIVER,  .        .        .        .        .        .  .        .18 

Considerations  in  Designing — Mechanical  and  Electrical  Efficiency — 
Single-Pole  Receivers — Bipolar  Receivers — Adjustment  between 
Magnet  and  Diaphragm — Material  for  Shells — Faults  of  Imitation 
Hard  Rubber — Commercial  Types  of  Receivers — Receiver  Cords — 
Details  of  Cord-Tip — Supports  for  Receiver  Cords. 

CHAPTER  IV. 
CARBON  TRANSMITTERS, 32 

Action  of  the  Transmitter— Single-Contact  Transmitters— Multiple-Con- 
tact Transmitters— Granular  Carbon  Transmitters — Commercial 
Types  of  Transmitters— Packing  :  Its  Remedy — Unusual  Forms  of 
Transmitters— Types  of  Carbon  Electrodes. 

CHAPTER  V. 
INDUCTION  COILS,    .  .........      53 

Advantages  of  the  Induction  Coil — Primary  Current — Secondary  Cur- 
rent— Design  of  Induction  Coils — Methods  of  Making  Comparative 
Tests— Results  of  Comparative  Tests— Selection  of  Coil  for  a  Trans- 
mitter—Commercial Coils — Varley  Method  of  Winding— Mounting 
of  Induction  Coils. 


COPYRIGHT,  1899,  BY 
AMERICAN  ELECTRICIAN  COMPANY. 


PREFACE. 


THE  intended  scope  of  this  book  is  set  forth  in  its  title.  To 
those  interested  the  writer  has  endeavored  to  present  in  as  clear 
a  manner  as  possible  the  general  principles  of  telephony,  the 
design  and  construction  of  commercial  apparatus,  the  circuits 
connecting  such  apparatus  into  operative  systems,  and  .the 
methods  used  in  the  construction,  operation,  and  maintenance  of 
these  systems.  No  attempt  whatever  has  been  made  to  treat 
the  subject  from  its  purely  mathematical  standpoint,  that  being 
beyond  the  scope  of  this  work.  -  The  apparatus  and  methods 
of  both  Bell  and  Independent  companies  have  been  given  im- 
partial attention. 

The  writer  sincerely  thanks  his  friends,  Mr.  Wm.  H.  Donner 
and  Mr.  Wm.  R.  Mackrille,  for  their  many  suggestions  and 
untiring  labors  in  proof  reading,  and  also  Mr.  W.  D.  Weaver, 
editor  of  the  Electrical  World  and  Engineer,  for  his  interest 
and  assistance  throughout  the  entire  preparation  of  this  book. 

KEMPSTER  B.  MILLER. 


VI  TABLE   OF   CONTENTS. 

CHAPTER  VI. 

PAGE 

BATTERIES, 62 

Simple  Cell — Direction  of  Current — Positive  and  Negative  Poles — 
Materials  Best  Suited  for  Electrodes— The  LeClanche  Cell— The 
Fuller  Cell— Specifications  for  Standard  Fuller  Cell— The  Gravity 
Cell — Local  Action  in  Batteries— Amalgamation  of  Zinc — Storage 
Batteries — The  Setting  up  and  Operating  of  Storage  Batteries- 
Determination  of  Positive  and  Negative  Poles  of  Charging  Circuit- 
Density  of  Electrolyte— Advantages  of  the  Storage  Battery. 

CHAPTER  VII. 
CALLING  APPARATUS, 75 

Battery  Calls — The  Magneto-Generator — Action  of  Magneto-Generator— 
The  Polarized  Bell  or  Ringer — Design  of  Magneto-Generators— 
Construction  of  Armature  Core — Winding  of  Armature — Permanent 
Magnets — Form  of  Current  Wave — Design  of  Polarized  Bells— Iron 
for  Ringer  Frames — Length  of  Hammer  Rod. 

CHAPTER  VIII. 

THE  AUTOMATIC  SHUNT, 86 

Necessity  for  the  Automatic  Shunt — Commercial  Types. 

CHAPTER  IX. 
THE  HOOK-SWITCH  AND  CIRCUITS  OF  A  TELEPHONE,  90 

Functions  of  the  Hook-Switch — Simplified  Telephone  Circuits — The 
Warner  Hook-Switch — Other  Forms  of  Hook-Switches—Circuits  of 
a  Series  Telephone — Circuits  of  a  Bridging  Telephone — Battery  Call 
Instruments— Desk  Telephone— Wiring  of 'Telephones. 

CHAPTER  X. 
COMMERCIAL  CALLING  APPARATUS, 104 

Types  of  Generators  and  Ringers — Resistances  of  Armatures  and  Ringer 
Magnets  for  Different  Kinds  of  Work— Constantly  Driven  Genera- 
tors— Methods  of  Driving  Generators — Motor  Generators. 

CHAPTER  XI. 
THE  TELEPHONE  RELAY  OR  REPEATER, uS 

Simple  Relay  Circuit — Difficulties  in  Producing  a  Two- Way  Repeater — 
Circuits     of    Two-Way    Repeater — The      Erdman     Repeater — The 
Stone  Repeater. 


TABLE   OF   CONTENTS.  vit 

CHAPTER  XII. 

PAGE 

SELF-INDUCTION  AND  CAPACITY I24 

Ohm's  Law — Field  of  Force  about  Conductor— Electromagnetic  Induc- 
tion—Action between  Turns  of  the  Same  Coil — Impedance — Effects 
of  Self-induction  on  Undulatory  Currents — Charge  of  Electricity — 
Action  between  Like  and  Unlike  Charges— Electrostatic  Induction — 
Condensers — Capacity — Specific  Inductive  Capacity  of  Dielectrics — 
Specific  Inductive  Capacity  in  Telephone  Cables— Effect  of  Con- 
denser Bridged  across  Circuit — Effect  of  Capacity  on  Varying  Cur- 
rents— Trans-Oceanic  Telephony. 

CHAPTER  XIII. 
TELEPHONE  LINES, .        .        .136 

Grounded  Circuits — Noises  on  Grounded  Circuits — Causes  of  Line  Dis- 
turbances—Electromagnetic Induction — Electrostatic  Induction — 
Carty's  Experiments — Cross-Talk—Elimination  of  Cross-Talk — 
Transpositions — Ground-Return  Systems — Common-Return  Sys- 
tems— Location  of  Common-Return  Wire— Size  of  Common-Return 
Wire — Connection  of  Metallic  and  Grounded  Circuits — The  Repeat- 
ing Coil — Elimination  of  Local  Induction. 

CHAPTER  XIV. 
SIMPLE  SWITCH-BOARDS  FOR  SMALL  EXCHANGES, 153 

Manual  and  Automatic  Switch-Boards— Switch-Boards  for  Grounded  or 
Common-Return  Systems — Types  of  Drops  and  Jacks — Switch- 
Boards  for  Metallic  Circuits — Tubular  Drops. 

CHAPTER  XV. 
LISTENING  AND  RINGING  APPARATUS  FOR  SWITCH-BOARDS,  .        .        .163 

The  O'Connell  Key— The  Cook  Key— The  American  Key— Plug  Listen- 
ing and  Ringing  Devices — Plug-Socket  Listening  Key. 

CHAPTER  XVI. 
SELF-RESTORING  SWITCH-BOARD  DROPS, 173 

Electrically  Restoring  Drops— Mechanically  Restoring  Drops — Com- 
mercial Types. 

CHAPTER  XVII. 

COMPLETE  SWITCH-BOARDS  FOR  SMALL  EXCHANGES,  .        .        .        .183 

Arrangement  of  Switch-Board  Parts— Description  and  Operation  of 
Boards  in  Common  Use. 

CHAPTER  XVIII. 
LAMP-SIGNAL  SWITCH-BOARDS 192 

Advantages  of  Incandescent  Lamps  for  Signals — Lamp  Directly  in  Line 
Circuit — Lamp  in  Local  Circuit  Controlled  by  Relay — Pilot  Lamp- 
Life  of  Lamp  Signals. 


Vlll  TABLE    OF   CONTENTS. 

CHAPTER  XIX. 

PAGE 

THE  MULTIPLE  SWITCH-BOARD, 200 

Limitations  of  the  Simple  Switch-Board — General  Arrangement  of 
Apparatus  in  the  Multiple  Board — Tests  for  Busy  Lines — The  Series 
Multiple  Board:  Its  Disadvantages— The  Branch  Terminal  Multi- 
ple Board— Spring-Jacks  for  Multiple  Boards — The  Kellogg  Divided 
Multiple  Board. 

CHAPTER   XX. 
TRANSFER   SYSTEMS,        ......         .....     216 

Complexity  of  Multiple  Switch-Boards — The  Sabin  and  Hampton 
Express  System — Incoming  and  Outgoing  Trunk  Lines — Details  of 
Subscribers'  Circuits — Details  of  Trunking  Circuits — The  Clearing- 
out  Signals— Western  Telephone  Construction  Company's  Transfer 
System — The  Cook-Beach  Transfer  System — The  American  Trans- 
fer System. 

CHAPTER  XXI. 
COMMON  BATTERY  SYSTEMS 236 

Advantages— Series  Systems  for  Grounded  and  Metallic  Circuits — The 
Stone  System — The  Hayes  Repeating-Coil  System — Supplying  Cur- 
rent over  Two  Line  Wires  in  Multiple — Storage  Battery  at  Sub- 
scriber's Station.  Dean's  Thermopile  System — Carty's  Multiple- 
Transmitter  Circuit — Series-Transmitter  Circuit — Detailed  Descrip- 
tion of  Dean's  Common  Battery  System— Of  Scribner's  Common 
Battery  System — Hayes'  Systems  as  Applied  to  Multiple  Board 
— Scribner's  System  as  Applied  to  Multiple  Board— The  St.  Louis 
Bell  Exchange. 

CHAPTER  XXII. 
HOUSE  SYSTEMS, 265 

General  Plan  of  House  or  Intercommunicating  Systems— Common  Bat- 
tery House  Systems— The  Ness  Automatic  Switch — Circuits  of 
Holtzer:Cabot  System. 

CHAPTER  XXIII. 
PROTECTIVE  DEVICES,  27^ 


Static  Arresters — Fusible  Arresters — Thermal  Arresters — Heat  Coils — 
Combined  Thermal  and  Static  Arresters— The  Rolfe  Arrester. 

CHAPTER  XXIV. 
DISTRIBUTING  BOARDS,  281 

Functions  of  the  Distributing  Board— The  Hibbard  Board— The  Ford 
and  Lenfest  Board— Jumper  Wires. 


TABLE   OF  CONTENTS.  ix 

CHAPTER  XXV. 

p 
-         •         •     294 


PAGE 

PARTY  LINES — NON-SELECTIVE. 


Classification— The  Series  Party  Line— Generators  and  Ringers  for 
Party  Lines— Carty's  Bridged  Party  Line— Generators  and  Ringers 
for  the  Bridging  System — Induction  Coils  for  Bridged  Lines — Con- 
nection of  Switch-Board  Drops  to  Party  Lines— Lock-out  Systems. 

CHAPTER   XXVI. 
PARTY   LINES — STEP-BY-STEP  SELECTIVE  SIGNALING,  ....     308 

General  Operation — Early  Systems  of  Dickerson,  Anders,  and  Lock- 
wood — Bridging  System  of  Reid  and  McDonald. 

CHAPTER  XXVII. 
PARTY   LINES — SELECTIVE  SIGNALING  BY   STRENGTH   AND  POLARITY,         .     318 

Success  of  this  Method  in  Telegraphy— Early  work  of  Anders— Sabin 
and  Hampton's  Three-Station  Line — Hibbard's  Four-Station  Line — 
McBerty's  Four-Station  Line— Dean's  Eight-Station  Line— The 
Barrett-Whittemore-Craft  System. 

CHAPTER  XXVIII. 
PARTY  LINES — HARMONIC  SYSTEMS  OF  SELECTIVE  SIGNALING,       .        .        .     338 

Principles  Involved — Early  Experiments — Lighthipe  Bridged  System — 
Harter  System — Unsatisfactory  Development  of  Harmonic  Systems. 

CHAPTER  XXIX. 
WIRE  FOR  TELEPHONE  USE,  .........     347 

Iron,  Copper,  and  Aluminum  for  Conductors — Tensile  Strength — Con- 
ductivity— Resistance — Weight  per  Mile-Ohm — Circular  Wire 
Gauge — Micrometer  Gauge — The  Brown  and  Sharpe  Gauge — Other 
Wire  Gauges— Manufacture  of  Iron  Wire — Galvanizing — Tests  for 
Galvanizing — Grades  of  Iron  Wire — Specifications  for  Iron  Wire — 
Advantages  of  Copper  for  Wire — Specifications  and  Data  for  Copper 
Wire. 

CHAPTER  XXX. 
POLE-LINE  CONSTRUCTION, 360 

Woods  for  Poles— Treatment  of  Poles— Sizes  of  Poles — Number  of  Poles 
per  Mile— Height  of  Poles — Pole  Guards — Data  Cpncerning  Poles— 
Creosoting  and  Vulcanizing  Poles— Cross-Arms — Attaching  Cross- 
Arm  to  Pole— Insulators— Construction  Tools— Setting  Poles— Guy- 
ing— Terminal  Poles— Anchors— Pole  Braces— Methods  of  Tying 
and  Splicing — Transpositions. 


X  TABLE   OF  CONTENTS. 

CHAPTER  XXXI. 

PAGE 

OVERHEAD  CABLE  CONSTRUCTION, 389 

Necessity  for  Cables — Comparative  Cost  of  Cable  and  Bare-Wire  Con- 
struction— Rubber  Cables — Objections  to  Rubber  Cables — Saturated 
Core  Paper  Cables— Dry-Core  Paper  Cables — Lead  Sheaths  for 
Cables — Data  Concerning  Cables — Supporting  Strand  for  Cables — 
Cable  Hangers — Method  of  Suspending  Cables — Directions  for 
Splicing — Cable  Terminals — Directions  for  Making  Pot-Head 
Terminals. 

CHAPTER  XXXII. 
UNDERGROUND  CABLE  CONSTRUCTION, 409 

Requirements  for  Conduits — Economy  of  Space — Wooden  Conduits — 
Clay  Conduits — Cement-Lined  Pipe — Cement  Arch  Conduit — 
Methods  of  Laying  Conduits— Man-Holes— Rodding — Drawing  in  of 
Cables — Steam  Power  for  Drawing  in — Gas  in  Man-Holes — Elec- 
trolysis— Determination  of  Danger  Points — Prevention  of  Elec- 
trolysis. 

CHAPTER  XXXIII. 
TESTING, 424 

Classification  of  Tests— Rough  Tests— Magneto  Testing  Set— Tests  for 
Grounds  and  Crosses  with  Magneto — Receiver  Test  for  Grounds  and 
Cosses — Continuity  Test — Quantitative  Measurements — Measure- 
ment of  Resistance— The  Wheatstone  Bridge — Directions  for  Operat- 
ing Bridge— The  Thomson  Galvanometer— The  D'Arsonval  Gal- 
vanometer—Advantages of  the  D'Arsonval  Galvanometer— The  Gal- 
vanometer Shunt — Taking  of  Galvanometer  Constant — Insulation 
Tests— Capacity  Tests— Location  of  Faults— The  Varley  Loop  Test. 


AMERICAN  TELEPHONE  PRACTICE. 


CHAPTER  I. 

HISTORY  AND  PRINCIPLES  OF  THE  MAGNETO  TELEPHONE. 

THE  history  of  the  telephone,  from  its  inception  to  its  present 
state  of  perfection,  is  interesting  in  the  extreme,  and  affords  a 
striking  example  of  the  fact  that  great  inventions  are  almost  in- 
variably the  result  of  long  and  careful  study  on  the  part  of  many 
workers,  rather  than  the  sudden  inspiration  of  a  single  genius, 
It  is  of  even  greater  interest  from  a  scientific  standpoint,  for  in 
no  way  can  one  obtain  a  better  idea  of  the  fundamental  princi- 
ples involved  in  telephony  than  by  following  their  development, 
step  by  step,  noting  the  contributions  made  by  each  of  the  many 
scientists  and  inventors  whose  names  are  closely  connected  with 
electrical  progress. 

These  steps  were  made  in  logical  order,  the  knowledge  con- 
tributed by  each  investigator  making  possible  a  deeper  insight 
into  the  subject  on  the  part  of  his  successors.  It  is  best,  there- 
fore, to  follow  this  order  in  obtaining  primary  ideas  of  the 
subject. 

The  history  of  the  knowledge  of  electromagnetism  begins  with 
July  20,  1820,  and  with  this  date  very  properly  begins  the  history 
of  the  electric  telephone.  On  that  day  Oersted,  a  professor  in 
the  University  of  Copenhagen,  discovered  that  a  magnetic  needle 
tends  to  place  itself  at  right  angles  to  a  wire  carrying  a  current 
of  electricity.  Ampere  immediately  took  up  the  subject,  and  in 
a  very  short  time  developed  the  laws  upon  which  present  electro- 
magnetic theory  is  based. 

In  the  following  year  Arago  and  Davy  discovered  that  if  a 
current  be  caused  to  flow  through  an  insulated  wire  wrapped 
about  a  rod  of  steel  the  latter  would  exhibit  magnetic  properties. 
It  was  William  Sturgeon,  however,  who  in  1825  made  an  electro- 
magnet as  we  know  it  to-day,  and  called  it  by  that  name.  To 
these  three  men,  therefore,  belongs  the  credit  of  one  of  the  greatest 
discoveries  in  the  history  of  science.  Joseph  Henry  also  made 


2  AMERICAN    TELEPHONE  PRACTICE. 

his  classic  experiments  on  the  electromagnet,  and  to  him  must 
be  accredited  a  large  amount  of  our  knowledge  regarding  it. 
Henry  showed  how  to  build  a  magnet  capable  of  being  operated 
over  a  great  length  of  wire,  a  most  important  step. 

In  1831  Faraday  and  Henry,  independently,  discovered  the  con- 
verse of  these  laws  of  electromagnetism — that  if  the  intensity  of 
a  magnetic  field  inclosed  by  a  conductor  be  in  any  wise 
changed,  a  current  of  electricity  will  flow  in  said  conductor. 
This  current  will  flow  only  while  such  change  is  taking  place, 
and  its  strength  will  depend  directly  on  the  rate  of  the  change. 

These  two  laws  concerning  the  transformation  of  electric 
energy  into  magnetic,  and  its  converse,  the  transformation  of 
magnetic  energy  into  electric,  are  certainly  the  most  important 
in  the  whole  realm  of  electrical  science  ;  as  singly  or  together 
they  form  the  foundations  not  only  of  the  telephone  and  tele- 
graph, but  of  electric  lighting,  electric  power  transmission,  and 


B 
Fig.  i.— Sturgeon-Electromagnet. 

of  every  other  achievement  by  which  electricity  has  revolution- 
ized the  methods  of  life  throughout  the  whole  civilized  world. 

As  these  laws  form  the  very  root  of  all  telephone  practice,  a 
few  illustrations  directly  in  line  with  the  principles  of  the  tele- 
phone will  not  be  amiss,  even  though  they  are  very  generally 
understood ;  for  they  will  give  a  clearer  understanding  of  the 
developments  made  by  subsequent  inventors.  If,  as  shown  in 
Fig.  i,  a  coil  of  wire  be  wrapped  around  a  rod,  J?,  of  iron  or 
steel,  and  a  battery,  B,  placed  in  circuit  with  the  coil,  the  rod 
becomes  a  magnet  upon  the  closure  of  this  circuit,  and  will 
attract  an  iron  armature,  A,  in  the  vicinity  of  either  of  its  poles. 
Any  variation  in  the  strength  of  this  current  will  cause  corre- 
sponding variations  in  the  attractive  power  of  the  magnet.  If 
the  rod  be  of  steel,  and  permanently  magnetized,  it  will  exert  an 
attractive  force  of  its  own  on  the  armature,  and  the  current  will, 
according  to  its  direction,  increase  or  diminish  this  attractive 
force. 

About  every  magnet  there  exists  a  field  of  force  ;  that  is,  a 
region  in  which  any  body  capable  of  being  magnetized  (such  as 


7 '//A'   MAGNETO    TELEPHONE.  3 

iron)  has  exerted  on  it,  by  the  magnet,  an  influence  of  attraction 
or  repulsion.  This  field  of  force  is  usually  graphically  repre- 
sented by  closed  curves,  radiating  from  the  poles  of  the  magnet, 
and  the  strength  of  the  magnet  is  commonly  measured  in  terms 
of  the  number  of  such  lines  radiating  from  one  of  its  poles.  A 
magnet  may  be  made  to  map  out  its  own  field  of  force  by  plac- 
ing it  in  a  horizontal  position  and  directly  over  it  a  sheet  of 


Pig.  2. — Lines  of  Force  of  Bar-Magnet. 

paper  or  cardboard.  If  iron  filings  are  then  dropped  from  a 
height  of  a  few  feet,  on  the  paper,  they  will  arrange  themselves 
in  the  direction  of  the  lines  of  force.  Fig.  2  shows  such  a  map 
produced  by  the  bar-magnet,  N  S. 

If  now  a  galvanometer,  G,  or  other  current-indicator  (Fig.  3) 
be  placed  in  circuit  with  a  coil,  C,  and  a  magnet,  N  S,  moved  in 


Fig.  3. — Faraday  and  Henry  Magneto-Electricity. 

the  vicinity  of  the  coil,  or  the  coil  in  the  vicinity  of  the  magnet, 
in  such  manner  as  to  change  the  number  of  lines  of  force  passing 
through  the  coil,  a  current  is  generated  in  the  coil  and  is  indi- 
cated by  the  galvanometer.  This  current  will  flow  only  while 


4  AMERICAN    TELEPHONE  PRACTICE. 

the  magnet  is  being  so  moved.  Its  direction  will  depend  on  the 
direction  of  the  lines  of  force  threading  the  coil  and  on  whether 
their  number  is  being  increased  or  diminished.  Its  strength  will 
depend  on  the  rate  at  which  their  number  is  changing. 

If  a  mass  of  iron  be  brought  within  the  field  of  a  magnet,  the 
field  becomes  distorted  by  virtue  of  a  larger  number  of  lines  find- 
ing their  path  through  the  space  occupied  by  the  iron  than 
through  the  same  space  when  filled  with  air.  Therefore,  if 
a  closed  coil  be  placed  about  a  pole  of  the  magnet  and  a  body  of 
iron  be  moved  to  and  from  the  pole,  the  intensity  of  the  field  in 
which  the  coil  lies  will  vary,  and  currents  of  electricity  will  flow 
in  the  coil. 

In  1837  Professor  Page  of  Salem,  Mass.,  discovered  that  a  rod 
of  iron,  suddenly  magnetized  or  demagnetized,  would  emit  certain 


P    K- 

f 

i  . 

Fig.  4. — Morse  Electromagnetic  Telegraph. 

sounds  due  to  a  molecular  rearrangement  caused  by  the  changing 
magnetic  conditions.  This  phenomenon  is  known  as  "  Page's 
effect." 

Late  in  the  thirties  Professor  S.  F.  B.  Morse  placed  at  one  end 
of  a  line  Sturgeon's  electromagnet,  M  (Fig.  4),  with  a  pivoted 
armature,  A,  and  at  the  other  end  a  battery,  B,  and  a  key,  K,  for 
making  and  breaking  the  circuit.  By  manually  closing  and  open- 
ing the  key,  the  core  of  the  magnet  became  magnetized  and  de- 
magnetized, thus  alternately  attracting  and  releasing  the  arma- 
ture. By  this  means  signals  were  sent  and  recorded  on  a  strip  of 
paper,  carried  on  a  roller,  R,  in  front  of  the  armature,  and  thus 
intelligence  was  practically  conveyed  by  electrical  means  be- 
tween distant  points. 

In  1854  a  Frenchman,  Charles  Bourseul,  predicted  the  trans- 
mission of  speech,  and  outlined  a  method  correct  save  in  one 
particular,  but  for  which  error  one  following  his  directions  could 
have  produced  a  telephone  of  greater  efficiency  than  that  sub- 
sequently devised  by  Bell.  His  words  at  this  date  seem  almost 
prophetic:  "  Suppose  a  man  speaks  near  a  movable  disk  suffi- 


THE   MAGNETO    TELEPHONE.  5 

ciently  flexible  to  lose  none  of  the  vibrations  of  the  voice,  and 
that  this  disk  alternately  makes  and  breaks  the  current  from  a 
battery  ;  you  may  have  at  a  distance  another  disk  which  will  sim- 
ultaneously execute  the  same  vibrations." 

Philip  Reis,  a  German  inventor,  constructed  a  telephone  in 
1861,  following  very  closely  the  path  outlined  by  Bourseul.  He 
mounted  a  flexible  diaphragm,  D  (Fig.  5)  over  an  opening  in  a 


Fig.  5. — Reis'  Make-and-Break  Telephone. 

wooden  box,  and  on  the  center  of  the  diaphragm  fastened  a  small 
piece  of  platinum,  P.  Near  this  he  mounted  a  heavy  brass  spring, 
s,  with  which  the  platinum  alternately  made  arid  broke  contact 
when  the  diaphragm  was  caused  to  vibrate.  These  contact  points 
formed  the  terminals  of  a  circuit  containing  a  battery,  B,  and 
the  receiving  instrument.  His  receiver  assumed  various  forms, 
prominent  among  which  was  a  knitting  needle,  Nt  wrapped  with 


Figs.  6.  and  7.— Reis'  Telephone  Transmitter  and  Receiver. 

silk-insulated  copper  wire  and  mounted  on  a  cigar  box  for  a 
sounding  board.  Its  operation  was  as  follows:  The  sound  waves 
set  up  by  the  voice  struck  against  the  diaphragm  of  the  trans- 
mitter, causing  it  to  vibrate  in  unison  with  them.  This  made  and 
broke  the  circuit  at  the  contact  points,  and  allowed  intermit- 
tent  currents  to  flow  through  the  receiver.  The  currents, 


6  AMERICAN   TELEPHONE  PRACTICE. 

which  exactly  synchronized  with  the  sound  waves,  caused 
a  series  of  sounds  in  the  knitting  needle  by  virtue  of  "  Page's 
effect."  The  sounding  board  vibrated  in  unison  with  the  mo- 
lecular vibrations  of  the  needle,  and  the  sound  was  thus  greatly 
amplified.  Reis'  transmitter  and  one  form  of  his  receiver  are 
shown  in  Figs.  6  and  7  respectively. 

Reis'  telephone  could  be  depended  upon  to  transmit  only 
musical  sounds,  but  it  is  probable  that  it  did  actually  transmit 
articulate  speech.  The  cause  of  this  partial  failure  will  be  under- 
stood from  the  following  facts : 

A  simple  musical  tone  is  caused  by  vibrations  of  very  simple 
form,  while  sound  waves  produced  by  the  voice  are  very  complex 
in  their  nature.  These  two  forms  of  waves  are  shown  graphically 
in  Fig.  8. 

Sound  possesses  three  qualities:  pitch,  depending  entirely  on 
the  frequency  of  the  vibrations;  loudness,  depending  on  the 


Fig.  8. — Sound  Waves  of  Voice  and  Simple  Musical  Note. 

amplitude  of  the  vibrations,  and  timbre  or  quality,  depending  on 
the  form  of  the  vibration.  The  tones  of  a  flute  and  a  violin  may 
be  the  same  as  to  pitch  and  loudness  and  yet  be  radically  different. 
This  difference  is  in  timbre  or  quality. 

Reis'  transmitter,  as  he  adjusted  it,  was  able  only  to  make 
and  break  the  circuit,  and  a  movement  of  the  diaphragm  barely 
sufficient  to  break  the  circuit  produced  the  same  effect  as  a  much 
greater  movement.  The  current  therefore  flowed  with  full 
strength  until  the  circuit  was  broken,  when  it  stopped  entirely. 
The  intermediate  strengths  needed  for  reproducing  the  delicate 
modulations  of  the  voice  were  entirely  wanting.  This  apparatus 
could  therefore  exactly  reproduce  the  pitch  of  a  sound,  but  not 
its  timbre  and  relative  loudness. 

For  the  next  fifteen  years  no  great  advance  was  made  in  the 
art  of  telephony,  although  many  inventors  gave  it  their  careful 
attention. 

In  1876  Professor  Alexander  Graham  Bell  and  Professor 
Elisha  Gray  almost  simultaneously  invented  successful  speaking 


THE  MAGNETO    TELEPHONE.  7 

telephones.  Bell  has,  however,  apparently  reaped  the  profit,  the 
U.  S.  Patent  Office  having  awarded  priority  of  invention  to  him. 
Bell  possessed  a  greater  knowledge  of  acoustics  than  of  electri- 
cal science,  and  it  was  probably  this  that  led  him  to  appreciate 
wherein  others  had  failed.  His  instrument  consisted  of  a  per- 
manent bar-magnet,  B  (Fig.  9),  having  on  one  end  a  coil  of  fine 
wire.  In  front  of  the  pole  carrying  the  coil  a  thin  diaphragm, 
D,  of  soft  iron  was  so  mounted  as  to  allow  its  free  vibration  close 


Fig.  9. — Bell-Magneto-Telephone. 

to  the  pole.     Two  of  the  instruments  are  shown  connected  in  a 
circuit  in  Fig.  9. 

Two  points  will  be  noticed  which  have  heretofore  been  absent : 
that  no  battery  is  used  in  the  circuit,  and  that  the  transmitting 
and  receiving  instruments  are  exactly  alike.  When  the  soft-iron 
diaphragm  of  the  transmitting  instrument  is  spoken  to,  it  vibrates 
in  exact  accordance  with  the  sound  waves  striking  against  it. 
The  movement  of  the  diaphragm  causes  changes  in  the  magnetic 
field  in  which  lies  the  coil,  which  changes,  as  shown  above,  cause 
an  alternating  current  to  flow  in  the  circuit.  This  current  varies 
in  unison  with  the  movements  of  the  diaphragm.  The  waves  of 
this  current  are  very  complex,  and  represented  graphically  are 
similar  to  those  of  the  voice  shown  in  Fig.  8.  Passing  along  the 
line  wire,  these  electrical  impulses,  so  feeble  that  only  the  most 
delicate  instruments  can  detect  them,  alternately  increase  and  de- 


Figs.  10.  and  ii. — Bell's  Early  Receiver  and  Transmitter. 

crease  the  strength  of  the  permanent  magnet  of  the  receiving  in- 
strument, and  thereby  cause  it  to  exert  a  varying  pull  on  its  soft- 
iron  diaphragm,  which,  as  a  result,  takes  up  the  vibrations  and 
reproduces  the  sound  faithfully.  Bell's  earlier  instruments,  ex- 
hibited in  1876  at  the  Centennial  in  Philadelphia,  are  shown 
in  Figs.  10  and  n,  the  former  being  his  receiver,  the  latter  his 


8  AMERICAN    TELEPHONE  PRACTICE. 

transmitter.  The  receiver  consisted  of  a  tubular  magnet  com- 
posed of  a  coil  of  wire  surrounding  a  core,  and  inclosed  in  an 
iron  tube.  This  tube  was  closed  by  a  thin  iron  armature 
or  diaphragm  as  shown.  The  transmitter  consisted  of  an 
electromagnet,  in  front  of  which  was  adjustably  mounted  a 
diaphragm  of  gold-beater's  skin  carrying  a  small  iron  armature  in 
its  center. 

Bell's  instrument,  in  a  modified  form,  is  the  standard  of  to-day. 
It  is  now  used  as  a  receiver  only,  a  more  efficient  transmitter,  de- 
pending upon  entirely  different  principles,  having  been  invented. 

In  speaking  of  Bell's  invention  Lord  Kelvin  has  said :  "  Who 
can  but  admire  the  hardihood  of  invention  which  devised  such 
very  slight  means  to  realize  the  mathematical  conception  that  if 


Fig.  12.— Royal  E.  House's  Electro-Phonetic  Telegraph. 

electricity  is  to  convey  all  the  delicacies  of  quality  which  dis- 
tinguish articulate  speech,  the  strength  of  its  current  must  vary 
continuously  as  nearly  as  may  be  in  simple  proportion  to  the 
velocity  of  a  particle  of  air  engaged  in  constituting  the  sound  ?" 

A  very  interesting  fact,  and  one  which  might  have  changed 
the  entire  commercial  status  of  the  telephone  industry  is  that  in 
1868  Royal  E.  House  of  Binghamton,  N.  Y.,  invented  and  pat- 
ented an  "  electro-phonetic  telegraph,"  which  was  capable  of 
operating  as  a  magneto-telephone,  in  the  same  manner  as  the 
instruments  subsequently  devised  by  Bell.  House  knew  nothing 
of  its  capabilities,  however,  unfortunately  for  him.  The  in- 
strument- is  shown  in  Fig.  12,  and  is  provided  with  a  sound- 


THE  MAGNETO    TELEPHONE.  9 

ing  diaphragm  of  pine  wood  stiffened  with  varnish,  mounted  in 
one  end  of  a  large  sound-amplifying  chamber  so  formed  as  to 
focus  the  sound  waves  at  a  point  near  its  mouth,  where  the  ear 
was  to  be  placed  to  receive  them.  The  electromagnet  adapted 
to  be  connected  in  the  line  circuit  had  its  armature  connected  by 
a  rod  with  the  center  of  the  wooden  diaphragm  as  shown.  By 
this  means  any  movements  irriparted  to  the  armature  by  fluctua- 
ting currents  in  the  line  were  transmitted  to  the  diaphragm, 
causing  it  to  give  out  corresponding  sounds  ;  and  any  movements 
imparted  to  the  diaphragm  by  sound  waves  were  transmitted  to 
the  armature,  causing  it  to  induce  corresponding  currents  in  the 
line.  Two  of  these  instruments  connected  in  a  circuit  as  shown 
in  Fig.  9  would  act  alternately  as  transmitters  and  receivers  in 
the  same  manner  as  Bell's  instruments. 


CHAPTER  II. 

HISTORY    AND    PRINCIPLES    OF    THE     BATTERY     TRANSMITTER. 

IT  has  been  shown  that  in  order  to  transmit  speech  by  electric- 
ity it  is  necessary  to  cause  an  undulatory  or  alternating  current 
to  flow  in  the  circuit  over  which  the  transmission  is  to  be  effected, 
and  that  the  strength  of  this  current  must  at  all  times  be  in  exact 
accordance  with  the  vibratory  movements  of  the  body  producing 
the  sound. 

Bell's  transmitter  was  used  as  the  generator  of  this  current ;  as 
a  dynamo,  in  fact,  the  energy  for  driving  which  was  derived  from 
the  sound  waves  set  up  by  the  voice.  The  amount  of  energy  so 
derived  was,  however,  necessarily  very  small  and  the  current 
correspondingly  weak,  and  for  this  reason  this  was  not  a  practi- 
cal form  of  transmitter,  except  for  comparatively  short  lines. 

Elisha  Gray  devised  a  transmitter  which,  instead  of  generating 
the  undulatory  current  itself,  simply  served  to  cause  variation  in 
the  strength  of  a  current  generated  by  some  separate  source. 


B 


Fig.  13. — Gray's  Variable  Resistance  Transmitter. 

He  accomplished  this  by  mounting  on  his  vibrating  diaphragm, 
D  (Fig.  13),  a  platinum  needle,  n,  the  point  of  which  was  im- 
mersed in  a  fluid  of  rather  low  conductivity,  such  as  water.  The 
variable  distance  to  which  the  needle  was  immersed  in  the  fluid, 
due  to  the  vibration  of  the  diaphragm,  caused  changes  in  the  re- 
sistance of  the  path  through  the  fluid,  and  corresponding  changes 
in  the  strength  of  the  current  set  up  in  the  circuit  by  the  bat- 
tery, B.  Instead  of  making  and  breaking  the  circuit,  as  did  the 
transmitter  of  Reis,  this  instrument  simply  caused  variations  in 
the  resistance  of  the  circuit,  and  thereby  allowed  a  continuous 
but  undulatory  current  to  pass  over  the  line.  The  variations  in 
this  current  conformed  exactly  with  the  sound  waves  acting  upon 
the  diaphragm,  and  were,  therefore,  capable  of  reproducing  all 


THE   BATTERY    TRANSMITTER. 


II 


the  delicate  shades  of  timbre,  loudness,  and  pitch  necessary  in 
articulate  speech. 

Gray  embodied  in  this  apparatus  the  main  principle  upon 
which  all  successful  battery  transmitters  are  based,  but  it  was 
not  long  before  a  much  better  means  was  devised  for  putting 
it  into  practice. 

In  1877  Emile  Berliner  of  Washington,  D.  C.,  applied  for  a 
patent  on  a  transmitter  depending  upon  a  principle  previously 
pointed  out  by  the  French  scientist,  Du  Moncel,  that  if  the  pres- 
sure between  two  conducting  bodies  forming  part  of  an  electric 
circuit  be  increased,  the  resistance  of  the  path  between  them  will 
be  diminished,  and  conversely,  if  the  pressure  between  them  be 
decreased,  a  corresponding  increase  of  resistance  will  result. 

Berliner's  transmitter  is  shown  in  Fig.  14,  which  is  a  reproduc- 
tion of  the  principal  figure  in  his  now  famous  patent  and  in  which 


Fig.  14. — Berliner's  Transmitter. 

A  is  the  vibratory  diaphragm  of  metal,  against  the  center  of  which 
rests  the  metal  ball,  C,  carried  on  a  thumb-screw,  B,  which  is 
mounted  in  the  standard,  d.  The  pressure  of  the  ball,  C,  against 
the  plate,  A,  can  be  regulated  by  turning  the  thumb-screw.  The 
diaphragm  and  ball  form  the  terminals  or  electrodes  of  a  circuit, 
including  a  battery  and  receiving  instrument.  Figs.  15  and  16 
show  two  different  views  of  an  exact  duplicate  of  Berliner's  original 
model  as  filed  in  the  patent  office.  This  was  very  roughly  con- 
structed as  shown.  The  diaphragm  was  a  circular  piece  of  ordi- 
nary tin  and  the  contact-piece  a  common  blued-iron  wood  screw. 


12 


AMERICAN   TELEPHONE   PRACTICE. 


Figs.  15.  and  16.— Berliner's  Patent-Office  Model. 


THE  BATTERY   TRANSMITTER.  13 

The  action  of  this  instrument  is  as  follows:  when  the  dia- 
phragm is  vibrating,  the  pressure  at  the  point  of  contact,  a,  be- 
comes greater  or  less,  thus  varying  the  resistance  of  the  contact 
and  causing  corresponding  undulations  in  the  current  flowing. 

Soon  after  this  Edison  devised  an  instrument  using  carbon  as 
the  medium  for  varying  the  resistance  of  the  circuit  with  changes 
of  pressure.  EdisonXfirst  iype  of  carbon  transmitter  consisted 
simply  of  a  button  of  compressed  plumbago  bearing  against  a 
small  platinum  disk  secured  to  the  diaphragm.  The  plumbago 
button  was  held  against  the  diaphragm  by  a  spring,  the  tension 
of  which  could  be  adjusted  by  a  thumb-screw. 

A  form  of  Edison's  transmitter,  devised  by  George  M.  Phelps 
in  1878,  is  shown  in  Fig.  17.  The  transmitting  device  proper  is 


Fig.  17. — Phelps-Edison  Transmitter. 

shown  in  the  small  cut  at  the  right  of  this  figure,  and  is  in- 
closed in  a  cup-shaped  case  formed  of  the  two  pieces,  A  and  B, 
as  shown.  Secured  to  the  front  of  the  enlarged  head,  e,  of  the 
adjustment  screw,  E,  is  a  thin  platinum  disk,  Ft  against  which 
rests  a  cylindrical  button,  G,  of  compressed  lampblack.  A  plate 
of  glass,  /,  carrying  a  hemispherical  button,  K,  has  attached 
to  its  rear  face  another  platinum  disk,  H.  This  second  platinum 
disk  rests  against  the  front  face  of  the  lampblack  disk,  G,  and  the 
button,  K,  presses  firmly  against  the  center  of  the  diaphragm, 
D.  The  plates,  F  and  //,  form  the  terminals  of  the  transmitter, 
and  as  the  diaphragm,  Z>,  vibrates,  it  causes  variations  in  the 
pressure,  and  corresponding  changes  in  the  resistance  of  the  cir- 
cuit, thus  producing  the  desired  undulations  of  current. 

Professor  David  B.  Hughes  made  a  most  valuable  contribution 
tending  toward  the  perfection  of  the  battery  transmitter.  By  a 
series  of  interesting  experiments,  he  demonstrated  conclusively 
that  a  loose  contact  between  the  electrodes  no  matter  of  what 
substance  they  are  composed,  is  far  preferable  to  a  firm,  strong 
contact.  The  apparatus  used  in  one  of  his  earlier  experiments, 


AMERICAN   TELEPHONE   PRACTICE. 


made  in  1878,  is  shown  in  Fig.  18,  and  consists  simply  of  three 
wire  nails,  of  which  A  and  B  form  the  terminals  of  the  circuit 
containing  a  battery  and  a  receiving  instrument.  The  circuit  was 
completed  by  a  third  nail,  C,  which  was  laid  loosely  across  the 
other  two.  Any  vibrations  in  the  air  in  the  vicinity  caused  vari- 
ations in  the  intimacy  of  contact  between  the  nails,  and  corre- 


Fig.  1 8. —Hughes'  Nail  Microphone. 

spending  variations  in  the  resistance  of  the  circuit.  This  was  a 
very  inefficient  form  of  transmitter,  but  it  demonstrated  the  prin- 
ciple of  loose  contact  very  cleverly. 

It  was  found  that  carbon  was,  for  various  reasons,  by  far 
the  most  desirable  substance  for  electrodes  in  the  loose-contact 
transmitter,  and  nothing  has  ever  been  found  to  even  approach 
it  in  efficiency. 

Another  form  of  transmitter  devised  by  Hughes,  and  called  by 
him  the  microphone,  is  shown  in  Fig.  19.  This  consists  of  a 


JUULfiJM__ 


Fig.  19. — Hughes'  Carbon  Microphone. 

small  pencil  of  gas  carbon,  A,  pointed  at  each  end,  and  two 
blocks,  B  B,  of  carbon  fastened  to  a  diaphragm  or  sounding 
board,  C.  These  blocks  are  hollowed  out,  as  shown,  in  such  a 
manner  as  to  loosely  hold  between  them  the  pencil,  A.  The 
blocks,  B  B,  form  the  terminals  of  the  circuit.  This  instrument, 
though  crude/in  form,  is  of  marvelous  delicacy  and  is  well  termed 
microphone.  The  slightest  noises  in  its  vicinity,  and  even  those 
incapable  of  being  heard  by  the  ear  alone,  produce  surprising 


THE   BATTERY    TRANSMITTER. 


'5 


effects  in  the  receiving  instrument.  This  particular  form  of 
instrument  is,  in  fact,  too  delicate  for  ordinary  use,  as  any  jar  or 
loud  noise  will  cause  the  electrodes  to  break  contact  and  produce 
deafening  noises  in  the  receiver.  Nearly  all  carbon  transmitters 
of  to-day  are  of  the  loose- contact  type,  this  having  entirely  su- 
perseded the  first  form  devised  by  Edison,  which  was  then  sup- 
posed to  depend  on  the  actual1  resistance  of  a  carbon  block  being 
changed  under  varying  pressure. 

Only  one  radical  improvement  now  remains  to  be  recorded.  In 
1 88 1  Henry  Runnings  devised  a  transmitter  wherein  the  variable 
resistance  medium  consisted  of  a  mass  of  finely  divided  carbon 
granules  held  between  two  conducting  plates.  His  transmitter 
is  shown  in  Fig.  20.  Between  the  metal  diaphragm,  A,  and  a 
parallel  conducting  plate,  B,  both  of  which  are  securely  mounted 
in  a  case  formed  by  the  block,  D,  and  a 
mouthpiece,  Fy  is  a  chamber  filled  with 
fine  granules  of  carbon,  C.  The  dia- 
phragm, A,  and  the  plate,  B,  form  the 
terminals  of  the  transmitter,  and  the  cur- 
rent from  the  battery  must  therefore  flow 
through  the  mass  of  granular  carbon,  C. 
When  the  diaphragm  is  caused  to  vibrate 
by  sound  waves,  it  is  brought  into  more 
or  less  intimate  contact  with  the  carbon 
granules  and  causes  a  varying  pressure 
between  them.  The  resistance  offered  by 
them  to  the  current  is  thus  varied,  and 
the  desired  undulations  in  the  current  pro- 
duced. This  transmitter,  instead  of  hav- 
ing  one  or  more  points  of  variable  contact, 
is  seen  to  have  a  multitude  of  them.  It 
can  carry  a  larger  current  without  heating,  and  at  the  same  time 
produce  greater  changes  in  its  resistance,  than  the  forms  previously 
devised,  and  no  sound  can  cause  a  total  break  between  the  elec- 
trodes. These  and  other  advantages  have  caused  this  type  in 
one  form  or  another  to  largely  displace  all  others.  Especially  is 
this  true  on  lines  of  great  length. 

Up  to  this  time  all  transmitters,  together  with  the  receiver  and 
battery,  had  been  put  directly  in  circuit  with  the  line  wire. 
With  this  arrangement  the  changes  produced  in  the  resistance 
by  the  transmitter  were  so  small  in  comparison  with  the  total 
resistance  of  the  circuit,  that  the  changes  in  current  were  also 
very  small,  and  produced  but  little  effect  on  the  receiver.  Edi- 


1 6  AMERICAN    TELEPHONE  PRACTICE. 

son  remedied  this  difficulty  by  using  an  induction  coil  in  connec- 
tion with  the  transmitter.  The  credit  of  this  improvement, 
however,  should  be  given  largely  to  Gray,  for  in  1875  he  had 
used  an  induction  coil  in  connection  with  his  harmonic  telegraph 
transmitter,  and  Edison  merely  substituted  a  telephone  transmit- 
ter in  the  circuits  used  by  Gray. 

The  induction  coil  used  then  and  now  is  made  as  follows: 
Around  a  core  formed  of  a  bundle  of  soft-iron  wires  is  wound  a 
few  turns  of  comparatively  heavy  insulated  copper  wire.  Out- 
side of  this,  and  entirely  separate  from  it,  is  wound  another  coil, 
consisting  of  a  great  number  of  turns  of  fine  wire,  also  of  copper 
and  insulated.  The  inner  coil  is  called  the  primary,  the  other 
the  secondary.  In  telephone  work  it  is  now  almost  universal 
practice  to  place  the  transmitter,  together  with  the  battery,  in  a 
closed  circuit  with  the  primary  of  the  induction  coil,  and  to 
place  the  secondary  directly  in  circuit  with  the  line  wire  and 
receiving  instrument.  This  is  shown  in  Fig.  21,  in  which  T 


Fig.  21. — Transmitter  with  Induction  Coil. 

is  a  transmitter,  B  a  battery,  P  and  S  the  primary  and  secondary, 
respectively,  of  an  induction  coil,  L  L  the  line  wires,  and  R  the 
receiving  instrument.  It  is  well  to  state  here  that  the  usual  way 
of  indicating  the  primary  and  secondary  of  an  induction  coil,  in 
diagraphic  representation  of  electrical  circuits,  is  by  an  arrange- 
ment of  two  adjacent  zigzag  lines,  as  shown  in  Fig.  20.  A  cur- 
rent flowing  in  the  primary  winding  of  the  induction  coil  pro- 
duces a  field  of  force  in  the  surrounding  space,  and  any  changes 
caused  by  the  transmitter  in  the  strength  of  the  current  produce 
changes  in  the  intensity  of  this  field.  As  the  secondary  winding 
lies  in  this  field,  these  changes  will,  by  the  laws  of  Faraday  and 
Henry,  cause  currents  to  flow  in  the  secondary  winding  and 
through  the  line  wire  to  the  receiving  instrument.  In  all  good 
induction  coils  the  electromotive  forces  set  up  in  the  secondary 
coil  bear  nearly  the  same  ratio  to  the  changes  in  electromotive 


THE   BATTERY   TRANSMITTER.  17 

force  in  the  primary  coil,  as  the  number  of  turns  in  the  second- 
ary bears  to  the  number  of  turns  in  the  primary. 

The  use  of  the  induction  coil  with  the  transmitter  accomplishes 
two  very  important  results  :  first,  it  enables  the  transmitter  to 
operate  in  a  circuit  of  very  low  resistance,  so  that  the  changes  in 
the  resistance  produced  by  the  transmitter  bear  a  very  large  ratio 
to  the  total  resistance  of  the  circuit.  This  advantage  is  well 
illustrated  by  contrasting  the  two  following  cases  : 

Suppose  a  transmitter  capable  of  producing  a  change  of  resist- 
ance of  one  ohm  be  placed  directly  in  a  line  circuit  whose  total 
resistance  is  1000  ohms ;  a  change  in  the  resistance  of  the  trans- 
mitter of  one  ohm  will  then  change  the  total  resistance  of  the 
circuit  one  one-thousandth  of  its  value,  and  the  resulting  change 
in  current  flowing  will  be  but  one  one-thousandth  of  its  value. 
On  the  other  hand,  suppose  the  same  transmitter  to  be  placed  in 
a  local  circuit  as  above  described,  the  total  resistance  of  which 
circuit  is  five  ohms  ;  the  change  of  one  ohm  in  the  transmitter 
will  now  produce  a  change  of  resistance  of  one-fifth  of  the  total 
resistance  of  the  circuit  and  cause  a  change  of  one-fifth  of  the 
total  current  flowing.  It  is  thus  seen  that  fluctuations  in  the 
current  can  be  produced  by  a  transmitter  with  the  aid  of  an 
induction  coil  which  are  many  times  greater  than  those  produced 
by  the  same  transmitter  without  the  coil. 

The  second  advantage  is  that  by  virtue  of  the  small  number 
of  turns  in  the  primary  winding  and  the  large  number  in  the 
secondary  winding  of  the  induction  coil,  the  currents  generated 
in  the  secondary  are  of  a  very  high  voltage  as  compared  with 
those  in  the  primary,  thus  enabling  transmission  to  be  effected 
over  much  greater  length  of  line  and  over  vastly  higher  resist- 
ances than  was  formerly  the  case. 


CHAPTER  III. 

THE    TELEPHONE    RECEIVER. 

To  construct  a  receiver  capable  of  reproducing  speech  is  a  very 
simple  matter.  In  fact,  nearly  any  electromagnet,  with  a  com- 
paratively light  iron  armature,  such  as  is  commonly  used  in 
electric  bells  and  telegraph  instruments,  may  be  made  to  repro- 
duce, with  more  or  less  distinctness,  sounds  uttered  in  the 
vicinity  of  a  transmitting  apparatus  with  which  it  is  in  circuit.  It 
has  proved  more  difficult,  however,  to  construct  a  receiving 
instrument  which  will  reproduce  speech  well,  and  at  the  same 
time  be  practically  successful  in  everyday  use. 

The  bar-magnet  with  a  thin  iron  diaphragm  in  close  proximity 
to  one  of  its  poles,  used  in  the  early  experiments  in  telephony, 
has  until  recently  been  very  generally  adhered  to  throughout 
this  country.  The  instrument  has  been  made  much  more  sensi- 
tive than  were  the  early  forms,  but  this  result  has  been  accom- 
plished by  better  mechanical  and  electrical  designs,  and  the  use 
of  better  materials,  and  not  by  any  departure  from  the  original 
principles  of  its  action. 

Aside  from  actual  talking  efficiency,  many  considerations  of  a 
purely  mechanical  nature  enter  into  the  design  of  a  good  tele- 
phone receiver.  It  should  be  durable  and  capable  of  with- 
standing the  rough  usage  to  which  it  will  necessarily  be  subjected 
by  careless  or  ignorant  users.  It  should  be  of  such  construction 
that  its  adjustment  will  not  be  changed  by  mechanical  shocks  or 
by  changes  in  temperature.  Failure  to  provide  against  this 
latter  effect  is  one  of  the  chief  sources  of  trouble  in  telephone 
work.  It  should  be  of  such  external  configuration  as  to  enable 
it  to  be  conveniently  placed  to  the  ear.  The  chamber  in  which 
the  diaphragm  vibrates  should  be  small  and  of  such  shape  as  not 
to  muffle  the  sound.  The  binding  posts  should  be  so  securely 
fastened  in  as  to  prevent  their  becoming  loose  and  twisting  off 
the  wires  inside  the  receiver  shell ;  and  the  construction  should 
be  so  simple  as  to  render  the  replacing  of  any  damaged  part  an 
easy  matter. 

By  far  the  greater  number  of  receivers  used  in  America  are  of 
the  single-pole  type ;  although  in  a  few  years  this  statement  will 


THE    TELEPHONE   RECEIVER. 


probably  not  be  true.  The  particular  form  shown  in  Fig.  22 
has  proved  efficient,  and  is  now  largely  used  by  the  American 
Bell  Telephone  Company.  Its  chief  merit  lies  in  its  simplicity. 

In  Fig.  22,  M  is  a  compound  bar-magnet,  composed  of  two 
pairs  of  separately  magnetized  steel  bars  arranged  with  like  poles 
together.  Between  the  pairs  pf  bars  is  clamped  a  soft-iron  pole- 
piece,  P,  at  one  end,  and>a  similarly  shaped  iron  block,  <2,  at  the 
other  end.  These  parts  are  firmly  bound  together  by  the  two 
screws,  5  5.  On  the  end  of  the  pole-piece  is  slipped  a  coil  of 
wire,  G.  This  coil  is  usually  wound 
with  two  parallel  No.  38  B.  &  S. 
silk-insulated  copper  wires,  and  has 
a  total  resistance  of  about  75  ohms. 

The  magnet  is  incased  in  a  shell 
of  hard  rubber,  composed  of  two 
pieces,  A  and  B,  which  screw  to- 
gether and  clamp  between  them  the 
diaphragm,  D,  of  thin  sheet  iron. 
The  piece,  B,  is  hollowed  out  as 
shown,  to  form  a  convenient  ear- 
piece. A  tailpiece,  T,  carrying  two 
binding  posts,  J  J,  fits  over  the  end 
of  the  case  opposite  the  earpiece,  B, 
and  is  held  in  place  by  a  screw,  E. 
This  screw  engages  a  threaded  hole 
in  the  block,  Q,  and  serves  not  only 
to  hold  the  tail-piece  in  place,  but 
to  bind  the  magnet  securely  to  the 
shell.  Soldered  to  the  binding  posts 
are  heavy  leading-in  wires,  W  W, 
which  pass  along  the  sides  of  the 
magnet  and  are  soldered  to  the  re- 
spective terminals  of  the  fine  wire 
forming  the  coil. 

The    diaphragm     of    this     instru- 
ment is  about  Yjy  in  thickness  and  2j"  in  diameter.     The  diam- 
eter of  the  free  portion  is  if". 

In  some  single-pole  receivers  the  old  style  of  magnet,  consist- 
ing of  a  single  cylindrical  bar  of  steel,  is  still  used  instead  of  the 
compound  magnet  formed  of  several  separately  magnetized  bars, 
but  with  generally  inferior  results,  owing  to  its  weaker  and  less 
permanent  magnetic  field. 

In  bipolar  receivers,  which  are  now  coming  into  general  use, 


Fig.  22.— Bell  Single-Pole 
Receiver. 


20 


AMERICAN    TELEPHONE  PRACTICE. 


the  object  is  to  strengthen  the  field  in  which  the  diaphragm  vi- 
brates, by  presenting  both  magnet  poles  to  the  diaphragm.  The 
length  of  the  path  of  the  lines  of  force  through  the  air  is  thus 
greatly  shortened,  and  the  field  of  force  is  concentrated  at  the 
point  where  it  will  be  most  effective. 

One  form  of  bipolar  receiver  is  shown  in  Fig.  23,  which  illustrates 
the  receiver  manufactured  until  recent  date  by  one  of  the  large 
independent  companies.  The  shell,  A,  and  ear-piece,  B,  are  of  a 


Fig.  23. — Bipolar  Receiver. 

material  resembling  hard  rubber,  and  clamp  between  them   the 
soft-iron  diaphragm,  D,  as  in  the  instrument  described  above. 

The  magnet  consists  of  two  pairs  of  separately  magnetized  steel 
bars,  F  F  and  F'F',  the  separate  bars  in  each  pair  being  laid  with 
like  poles  together,  so  that  each  pair  forms  in  itself  a  compound 
bar-magnet.  These  two  compound  bar-magnets  are  so  laid 
together  that  the  north  pole  of  one  is  opposite  the  south  pole  of 
the  other.  The  two  pairs  of  bars  are  held  apart  at  one  end  by 
the  adjustment  block,  H,  made  of  the  same  material  as  the  shell, 


THE    TELEPHONE  RECEIVER.  21 

and  at  the  other  end  by  the  soft-iron  block,  7.  On  each  side  of 
the  block,  H,  and  between  it  and  the  pairs  of  bar-magnets,  are 
the  soft-iron  pole-pieces,  P  P,  on  which  are  wound  the  coils  G  G, 
having  a  resistance  of  50  ohms  each.  These  coils  are  wound 
with  No.  36  B.  &  S.  silk-insulated  wire  and  are  connected  in 
series,  so  that  the  total  resistance  of  the  receiver  is  100  ohms. 

The  block,  H,  has  two  segmental  flanges  projecting  out  beyond 
the  sides  of  the  magnet  bars.  These  flanges  are  screw-threaded 
on  their  circumferential  surfaces  so  as  to  engage  a  thread,  g,  on 
the  inner  surface  of  the  shell,  A.  The  magnet  may  thus  be 
adjusted  toward  or  from  the  diaphragm  by  turning  it  in  the 
shell,  A. 

A  tail-piece,  T,  of  hard  rubber  is  so  shouldered  as  to  fit  into  the 
small  end  of  the  receiver  shell,  and  is  prevented  from  turning  in 
its  place  by  small  lugs  fitting  into  notches  in  the  shell.  A  screw, 
E,  extends  through  the  tail-piece  and  clamps  the  magnet  into  any 
position  to  which  it  has  been  adjusted.  To  the  binding  posts, 
JJ,  are  soldered  heavy  leading-in  wires,  W  W,  which  pass  through 
holes  in  the  adjustment  block,  H,  and  are  soldered  to  the  termi- 
nals of  the  fine  magnet  wire.  These  heavy  wires,  W  W,  are 
firmly  knotted  after  passing  through  thetblock,  H,  in  order  to 
prevent  any  mechanical  strain  coming  on  the  hair-like  wires  of 
the  magnet  coils  when  the  tail-piece  is  removed.  Sufficient 
slack  is  left  in  the  leading-in  wires  to  allow  the  removal  of  the 
tail-piece  a  short  distance,  to  give  access  to  the  end  of  the 
magnet  for  purposes  of  adjustment. 

In  many  forms  of  receiving  instruments  much  trouble  is 
experienced  in  keeping  permanent  the  adjustment  between  the 
magnet  and  the  diaphragm.  This  is  due  to  the  fact  that  steel 
and  hard  rubber  differ  widely  as  to  their  amounts  of  expansion 
or  contraction  under  changes  in  temperature.  In  instruments 
where  the  magnet  is  rigidly  secured  to  the  shell  only  at  a  point 
at  considerable  distance  from  the  diaphragm,  the  unequal  expan- 
sion or  contraction  of  the  magnet  and  the  shell  causes  the  dis- 
tance between  the  pole-piece  and  the  diaphragm  to  vary  with 
every  change  in  temperature.  A  sudden  change  will  thus  often 
render  a  receiver  inoperative. 

This  defect  is  seen  to  exist  without  any  attempt  at  a  remedy 
in  the  single-pole  receiver  shown  in  Fig.  22.  The  point  of 
support  of  the  magnet  is  as  far  removed  from  the  diaphragm  as. 
possible,  being  at  the  screw,  E,  and  therefore  the  full  benefit 
(which  is  of  course  negative)  of  all  the  differences  in  contraction 
and  expansion  between  the  hard  rubber  and  the  steel  is  obtained. 


22  AMERICAN   TELEPHONE  PRACTICE. 

In  the  receiver  shown  in  Fig.  23  an  attempt  was  made  to 
remedy  this  defect  by  securing  the  magnet  to  the  shell  at  a  point 
close  to  the  diaphragm,  so  that  the  differences  in  expansion  and 
contraction  between  the  shell  and  magnet  will  be  reduced  to  a 
minimum.  This,  however,  in  this  particular  case  introduced  a 
defect  quite  as  serious,  because  the  shell  was  also  bound  to  the 
magnet  by  the  screw,  E.  The  contraction  and  expansion  thus 
tended  to  loosen  the  screw-thread  on  the  block,  H,  making  fre- 
quent readjustment  necessary.  Moreover,  a  good  screw-driver  in 
the  hands  of  an  ordinary  repair  man  or  of  a  subscriber  often  sub- 
jects the  screw-thread  on  block,  //,  to  such  a  strain  as  to  strip  the 
thread,  thus  rendering  the  receiver  useless. 

Several  important  lessons  may  be  and  have  been  learned  from 
the  behavior  of  these  two  forms  of  receiver  in  actual  and  long- 
continued  service  : 

First :  It  is  poor  construction  to  secure  the  magnet  in  the 
shell  at  the  end  farthest  from  the  diaphragm. 

Second  :  It  is  also  poor  construction  to  secure  it  rigidly  near 
the  diaphragm  and  also  at  the  opposite  end. 

Third  :  It  is  extremely  poor  construction  to  use  any  of  the 
materials  imitating  hard  rubber  in  vital  portions  of  the  instru- 
ment. These  materials,  so  far  produced,  are  without  exception 
subject  to  some  or  all  of  the  following  faults  to  a  greater  extent 
than  hard  rubber,  viz.:  They  are  not  sufficiently  tough,  and  are 
usually  very  brittle.  They  absorb  moisture.  They  soften  when 
exposed  to  heat,  and  gradually  give  way  under  pressure,  causing 
them  to  retain  a  permanent  set  when  again  cooled.  They  are 
capable  of  having  threads  molded  upon  them,  and  as  a  rule 
these  molded  threads  do  not  fit.  Threads  in  hard  rubber  are 
cut,  and  may  therefore  be  as  accurate  as  desired.  They  are  liable 
to  have  seams  or  "  cold  shuts  "  formed  in  molding  which  will 
cause  cracks  and  fractures;  and  lastly:  They  are  not  as  good 
insulating  materials  as  hard  rubber.  Some  of  these  materials 
may  not  possess  all  of  these  objections,  but  all  possess  some  of 
them.  Hard  rubber  therefore  is,  so  far  as  materials  are  at  present 
developed,  the  only  thing  to  use  in  the  insulating  portions  of 
receiver  shells. 

A  way  of  obviating  the  expansion  and  contraction  difficulty, 
used  largely  in  European  countries  and  to  an  increasing  extent 
in  this  country,  is  to  construct  the  shell  holding  the  diaphragm 
of  some  metal  having  nearly  the  same  coefficient  of  expansion 
as  thec  steel  magnets. 

Fig.  24  shows    one  of   the  early   forms   of    bipolar   receivers. 


THE    TELEPHONE  RECEIVER.  23 

This  was  devised  in  1881  by  Clement  Ader  of  Paris,  France,  and 
is  with  some  modifications  largely  used  in  France  and  other 
European  countries  to-day.  This  embodies  the  results  of  one  of 
the  few  successful  attempts  at  increasing  the  electrical  efficiency 
of  the  telephone  receiver.  The  magnet,  B,  is  ring-shaped,  and 
has  fastened  to  its  poles  two  L-shaped  pole-pieces  carrying  coils, 
C  C.  The  box,  R,  inclosing  the  pole-pieces  and  coils  is  of  brass 
and  is  secured  to  the  magnet  by  screws,  E  E.  It  is  screw- 
threaded  at  G,  so  as  to  engage  a  corresponding  screw-thread  on 
the  inner  surface  of  the  cap,  5,  which  has  a  flaring  portion,  H, 


Fig.  24.  — Ader  Bipolar  Receiver. 

forming  an  ear-piece.  The  diaphragm,  D,  is  clamped  between 
the  pieces,  R  and  5,  as  in  the  American  instruments  described 
above. 

Surrounding  the  opening,  leading  from  the  diaphragm  to  the 
ear-piece,  is  a  ring,  m,  of  soft  iron,  and  in  this  ring  lies  the  chief 
point  of  Ader's  invention.  The  additional  mass  of  iron  placed 
near  the  poles  of  the  magnet  affords  a  more  ready  path  for  the 
lines  of  force,  and  their  number  is  thus  increased.  The  dia- 
phragm, therefore,  moves  in  a  stronger  field  of  force,  and  the 
power  of  the  receiver  is  said  to  be  correspondingly  augmented. 
Practice  in  this  country  has  not,  however,  shown  any  perceptible 
gain  of  efficiency  by  the  use  of  this  ring. 


24  AMERICAN    TELEPHONE  PRACTICE. 

Fig.  25  shows  the  form  of  receiver  now  manufactured  by 
the  Western  Telephone  Construction  Company  of  Chicago.  In 
this  the  shell  is  of  hard  rubber,  composed  of  three  pieces  ;  the 
diaphragm  being  clamped  between  the  shell  and  ear-piece  in  the 
ordinary  manner.  The  magnet  is  of  horseshoe  form  and  carries 
a  block  of  brass  grooved  on  each  side  to  partially  inclose  the 
magnet  limbs.  The  lower  portion  of  this  block  is  screw-threaded, 
as  shown,  so  as  to  engage  the  corresponding  thread  turned  in 
the  receiver  shell.  The  upper  flange  on  the  block  rests  on  a 
corresponding  flange  on  the  interior  of  the  shell  when  the  magnet 
is  screwed  home.  The  pole-pieces  are  secured  to  the  outside  of 
the  magnets  by  a  bolt  passing  entirely  through  the  brass  block, 
each  limb  of  the  magnet,  and  each  pole-piece.  This  bolt  is  pro- 
vided with  a  nut  at  each  end,  so  that  either  pole-piece  may  be 
taken  off  without  removing  the  other.  The  heads  of  the  magnet 


Fig.  25.— Western  Telephone  Construction  Co.'s  Receiver. 

spools  are  of  brass  pressed  into  position  on  the  pole-pieces. 
After  being  insulated,  the  spools  so  formed  are  wound  in  a 
machine  having  a  special  chuck  for  holding  the  pole-pieces.  The 
two  coils  are  for  standard  work,  wound  to  a  resistance  of  50  ohms 
each  with  No.  36  silk-insulated  wire  and  connected  in  series, 
thus  making  the  total  resistance  of  the  receiver  100  ohms. 

The  novelty  in  this  receiver  is  in  the  method  of  attaching  the 
receiver  cord  to  the  terminals  leading  from  the  coils.  These 
terminals,  as  shown,  are  composed  of  heavily  insulated  wire  pass- 
ing through  the  brass  block  into  the  coil  chamber.  The  other 
ends  of  these  wires  are  twisted  together  and  pass  through  a  cen- 
tral opening,  where  each  is  soldered  to  a  connector  held  in  place 
against  the  shell  by  a  small  machine  screw.  The  cord  is  provided 


THE    TELEPHONE  RECEIVER. 


with  similar  connectors,  which  may  be  slipped  under  the  screw- 
heads,  thus  completing  the  circuit  between  the  cord  and  the 
wires  of  the  receiver.  The  connection  of  the  cord  is,  of  course, 
made  before  the  tail-cap  is  screwed  in  place.  An  enlargement  in 
the  covering  of  the  cord  effectually  prevents  any  strain  ever 
coming  on  the  cord  terminals  when  the  receiver  is  dropped. 
Another  feature  secured^  by  this  construction  is  that  no  metal 
parts  are  exposed  on  the  outside  of  the  shell,  thus  insuring 
immunity  from  electric  shocks  while  handling  the  receiver,  this 
being  considered  very  desirable  by  some. 

This  receiver,  except  for  the  method  of  connecting  the  cord, 
which  was  designed  by  the  writer,  is  almost  identical  with  that 
used  by  The  American  Bell  Telephone 
Co.,  in  their  long-distance  work,  and 
also  in  most  of  their  common-battery 
exchanges.  In  the  Bell  receiver  the 
tail-cap  is  not  provided,  and  instead  two 
flanged  binding  posts  are  screwed  directly 
to  the  hard  rubber  fronr;  the  outside. 
To  these  binding  posts  the  ordinary 
receiver  cord  is  attached  in  the  usual 
way,  and  the  wires  leading  from  the 
receiver  magnets  pass  through  the  shell 
and  are  soldered  to  an  extension  of  the 
binding  posts.  These  forms  of  receiver 
are  very  efficient,  very  easily  adjusted, 
and  subject  to  little  or  no  trouble  from 
the  source  of  expansion  and  contraction, 
it  being  seen  that  the  magnet  is  sup- 
ported at  a  point  near  the  diaphragm 
without  being  bound  to  the  shell  at  any 
other  point. 

The  Stromberg-Carlson  receiver  is  a 
very  powerful  one,  and  has  stood  the  test  of  time  well.  It  is 
shown  in  Fig.  26,  in  which  a  is  a  casing  of  brass,  forming  a 
framework  upon  which  all  other  parts  of  the  instrument  are 
supported.  This  is  screw-threaded  on  its  outer  surface  to 
receive  the  internally  screw-threaded  cap,  b,  and  lock-ring,  b\ 
One  unique  feature  of  this  receiver  is  the  method  of  supporting 
the  diaphragm,  which  is  held  in  place  in  the  cap,  b,  by  the 
clamping  ring  b'. 

Upon  the  cap,  b,  is  screwed  an  ear-piece,  &\     The  lock-ring,  b\  is 
adapted  to  be  screwed  against  the  cap,  b,  to  lock  it  in  any  adjusted 


Fig.  26. — Stromberg-Carl- 
son Receiver. 


26 


AMERICAN    TELEPHONE  PRACTICE.. 


position.  Upon  the  rear  of  the  casing  is  provided  a  projection, 
a',  against  the  faces  of  which  rest  the  soft-iron  cores,  cl  <:3,  which 
extend  through  the  bottom  of  the  casing  and  carry  upon  their 
ends  the  telephone  coils,  c3  c\  The  ends  of  the  permanent 
magnet,  d,  rest  upon  the  cores,  cl  f,  and  a  screw  or  bolt,  e\  passes 
through  the  ends  of  the  magnet,  the  cores,  and  the  projection,  to 
maintain  them  in  position.  The  ends  of  the  magnet,  d,  are  cut 
away  as  shown  to  permit  the  cores  to  be  set  flush  with  the  inner 
faces  of  the  magnet. 

Between  the  limbs  of  the  magnet,  d,  is  provided  a  block,  d ',  of 


Fig.  27.— American  Electric  Telephone  Co.'s  Receiver. 

fiber  upon  which  are  mounted  two  binding  posts,  d*,  the  binding 
posts  being  connected  to  the  coils,  c*  c\  by  heavy  insulated 
wires,  d*  d\  To  the  binding  posts,  d*,  are  also  attached  the  ends 
of  the  receiver  cord.  Upon  the  rear  of  the  casing,  a,  is  provided 
a  threaded  flange  upon  which  the  insulating  casing,/,  is  screwed, 
this  latter  being  provided  with  an  opening  at  the  end  through 
which  the  receiver  cord  passes. 

The  magnet,  d,  is  mounted  rigidly  upon  the  casing,  «,  the  cas- 
ing,/,  being  entirely  independent  so  that  it  may  be  removed  by 
unscrewing.  The  diaphragm  support  or  cap,  b,  may  be  raised 
or  lowered  to  adjust  the  diaphragm  relatively  to  the  magnet 


THE    TELEPHONE   RECEIVER.  27 

cores,  c    c\  the  ring,  b\  serving  to  lock  the  diaphragm  in  its  ad- 
justed position. 

This  receiver  does  away  entirely  with  the  troublesome  effects 
due  to  expansion  or  contraction.  The  insulating  casing  forms 
a  handle  and  serves  as  a  protection  to  the  cord  terminals,  but 
forms  no  part  of  the  working  structure  itself. 

In  Fig.  27  is  shown  the  bipolar  receiver  of  the  American 
Electric  Telephone  Co.  The  permanent  magnet  is  formed  of  two 
pieces  which  clamp  between  them,  at  the  end  farthest  from  the 
diaphragm,  a  cast-iron  block  on  which  is  mounted  a  hard-rubber 
disk  carrying  the  binding  posts.  This  block,  therefore,  serves 
the  double  purpose  of  completing  the  magnetic  circuit  between 
the  ends  of  the  magnets  and  of  a  support  for  the  binding  posts 
and  connections.  The  lower  ends  of  the  magnets  carry  a  screw- 
threaded  disk  upon  which  is  screwed  the  metal  cup  containing 
the  coils  and  against  which  the  diaphragm  rests.  Upon,  this 
block  are  also  mounted  the  straight  pole-pieces  carrying 
coils  similar  to  those  shown  in  Fig.  23.  The  cup  forming  the 
chamber  for  the  coils  is  of  pressed  brass,,  nickel-plated  and  screw- 
threaded  to  engage  the  threaded  disk  carried  by  the  magnet. 
Over  this  entire  structure  is  slipped  the  case  of  hard  rubber,  the 
diaphragm  being  clamped  between  the  earpiece  and  the  brass  cup. 
In  this  receiver,  adjustment  is  obtained  by  turning  the  cup  on 
the  magnet,  the  screw-threads  producing  a  longitudinal  move- 
ment of  the  latter  in  respect  to  the  former,  thus  moving  the 
pole-pieces  toward  or  from  the  diaphragm,  according  to  the 
direction  of  the  rotation.  After  the  desired  adjustment  has 
been  obtained,  the  cup  may  be  clamped  in  the  position  desired 
by  two  screws  projecting  through  flanges  carried  by  the  circular 
disks  of  the  magnets  and  extending  into  the  interior  of  the  cup. 
These  screws,  when  set,  engage  the  bottom  of  the  cup  in  such 
manner  as  to  hold  it  from  turning. 

A  single-pole  receiver,  manufactured  by  the  Holtzer-Cabot 
Electric  Co.,  and  embodying  features  of  decided  merit,  is  shown 
in  Fig.  28.  In  this  the  magnet  is  composed  of  four  bars  of  steel 
separately  magnetized  and  clamping  between  them  at  one  end 
an  iron  block  drilled  and  tapped  to  receive  the  screw  passing 
through  the  end  of  the  shell.  A  pole-piece  flanged,  and  screw- 
threaded  as  shown,,  is  clamped  between  the  other  ends  of  the 
magnets.  The  cup  for  inclosing  the  coil  and  carrying  the 
diaphragm  is  of  brass,  having  a  hole  through  its  center,  screw- 
threaded  to  engage  the  threaded  portion  of  the  pole-piece. 
When  screwed  in  position,  the  flat  portion  of  the  cup  abuts  the 


28  AMERICAN   TELEPHONE  PRACTICE. 

flange  on  the  "pole-piece,  thus  binding  the  two  rigidly  together. 
The  ear-piece  is  of  hard  rubber,  as  usual,  and  screws  to  the 
brass  cap,  thus  holding  the  diaphragm  in  position.  The  inclos- 
ing shell  of  hard  rubber  slips  over  the  magnet  as  shown,  and 
carries  the  binding  posts,  which  are  connected  by  heavy  leading- 
in  wires  to  the  receiver  coil.  No  form  of  adjustment  is  pro- 
vided for  this  instrument,  and  this,  by  the  way,  is  a  feature 


Fig.  28. — Holtzer-Cabot  Receiver. 

which  is  meeting  with  considerable  favor  and  is  being  adopted 
by  several  manufacturing  companies.  Great  care  is  taken  by 
the  manufacturers  to  adjust  the  instrument  properly  before  it 
leaves  the  factory,  after  which,  with  an  instrument  properly  con- 
structed, no  need  for  adjustment  should  exist. 

Still  another  form  of  receiver,  and  one  of  the  non-adjustable 
type,  is  shown  in  Fig.  29:  this  is  manufactured  by  the  Erics- 
son Co.  of  Sweden,  and  is  being  imported  into  this  country  to 
a  considerable  extent.  This  is  of  the  bipolar  type  presenting 
to  the  diaphragm  two  coils  and  two  pole-pieces  very  similar 
in  shape  to  those  shown  in  Fig.  23.  The  magnets  are 
secured  to  the  metal  cup  by  means  of  two  screws  shown  in  the 


Fig.  29. — Ericsson  Receiver. 

figure,  each  extending  transversely  through  the  case  and  into 
the  magnets.  The  holes  in  the  case  through  which  these  screws 
project  are  slotted  so  that  a  certain  amount  of  adjustment  can 
be  obtained  if  it  is  absolutely  necessary,  although  the  idea  of  the 
manufacturers  is  to  bind  it  so  tightly  that  no  adjustment  will 
ever  be  needed.  The  inclosing  tube  for  the  magnets  is  of  brass 
covered  by  a  thin  layer  of  insulating  material,  usually  hard  rubber, 


UNIVERSITY 

^CALIFL 
THE    TELEPHONE  J&CE1  V^R.  29 

but  sometimes  of  leather.  This  tube  is  also  held  in  position  by 
the  screws  before  mentioned.  A  piece  of  hard  rubber  projects 
between  the  two  binding  posts  of  the  instrument  as  shown,  the 
object  of  this  being  to  prevent  the  tips  of  the  receiver  cords 
from  twisting  the  posts  in  their  sockets  until  they  touch  each 
other,  thus  short-circuiting  the  instrument.  This  same  feature 
will  be  noticed  in  the  t  receiver  shown  in  Fig.  27.  This 
receiver  is  extremely  well  made,  very  handsome  in  appearance 
and  very  efficient,  and  probably  would  have  come  into  very  large 
use  were  it  not  for  its  high  cost. 

In  the  receivers  shown  in  Figs.  26,  27,  28,  and  29  the  evil 
effects  due  to  contraction  and  expansion  of  the  various  parts  are 
avoided  by  the  use  of  metal  cups  for  securing  the  diaphragm. 
This  method,  as  before  stated,  is  coming  into  increasing  favor  in 
this  country,  although  it  had  long  been  used  in  Europe.  The 
first  receiver  built  with  a  metal  cup  which  came  into  anything 
like  extensive  use  in  this  country,  was  designed  by  Messrs. 
Stromberg  &  Carlson,  and  is  of  substantially  the  same  form  as 
that  shown  in  Fig.  26.  The  advocates  of  the  hard-rubber  shell 
claim  that  the  exposed  metal  portion  of  the  cup  is  a  source  of 
great  danger  in  lightning  storms,  or  in  case  the  line  has  become 
crossed  with  some  high-potential  wire.  This  idea  has  been 
carried  to  its  extreme  in  Fig.  25,  where  not  even  the  binding  posts 
are  exposed.  When  it  is  remembered,  however,  that  many  other 
parts  of  a  telephone,  such  as  the  line  binding  posts,  generator 
crank,  magneto-gongs,  transmitter,  and  transmitter  arm,  have 
exposed  metal  surfaces  some  of  which  are  directly  in  connection 
with  the  line,  it  is  somewhat  doubtful  whether  this  objection  is  a 
very  valid  one. 

The  diaphragms  used  for  receivers  are  made  of  very  soft  thin 
sheet  iron  ;  the  ferrotype  plate  formerly  used  for  tin-types  in 
photography  being  as  good  material  as  can  be  found  for  this  pur- 
pose. Some  companies,  however,  notably  the  Ericsson,  the 
Stromberg  &  Carlson,  and  the  American,  are  using  tinned  dia- 
phragms, which  give  equally  good  results. 

The  diaphragms  for  the  various  receivers  here  described  vary 
from  2  to  2T5¥  inches  in  diameter,  the  free  portions — that  is,  the 
portion  not  clamped  by  the  supports — ranging  from  if  to  2^- 
inches.  The  usual  thickness  is  from  .009  to  .01 1  of  an  inch.  The 
thickness  of  a  diaphragm,  to  produce  the  best  results  with  the  given 
receiver,  must  be  obtained  by  experiment,  as  it  depends  on  the 
diameter  of  the  portion  free  to  vibrate,  and  also  on  the  strength 
of  the  magnetic  field  due  to  the  permanent  magnet.  It  has  been 


30  AMERICAN    TELEPHONE  PRACTICE. 

shown  that  with  a  very  thin  diaphragm  and  a  very  powerful  magnet 
the  iron  in  the  diaphragm  becomes  saturated  so  that  it  is  not 
responsive  to  changes  in  the  strength  of  the  existing  field.  Of 
course,  the  thicker  the  diaphragm  is  the  less  likely  is  this  to  occur. 
Many  manufacturers  aim  at  making  the  magnets  of  their  receivers 
extremely  powerful,  but  it  is  very  doubtful  if  much  or  any 
increased  efficiency  results  therefrom. 

The  question  of  receiver  cords  is  one  of  a  good  deal  of  impor- 
tance, as  a  faulty  cord  is  one  of  the  most  prolific  sources  of  trouble 
of  any  part  of  a  telephone  instrument.  If  the  conductors  in  a 
cord  are  not  properly  insulated,  so  that  they  may  come  into  con- 
tact, or  if  a  break  occurs  in  one  of  the  conductors,  the  instrument 
will  be  short-circuited  in  the  one  case,  or  the  circuit  left  open  in 
the  other.  In  either  event  the  receiver  is  rendered  completely 
useless.  These  faults  are  frequently  very  elusive,  as  a  slight 
movement  of  the  cord  may  cause  them  to  appear  or  disappear. 
The  conductors  in  receiver  cords  are  usually  composed  of  tinsel 
woven  or  twisted  into  strands,  and  a  few  strands  of  fine  cop- 


Fig.   30. — Details  of  Receiver  Cord  Tip. 

per  wire  are  frequently  added  to  give  greater  strength.  These 
tinsel  conductors  are  then  tightly  braided,  or  wrapped  with 
cotton,  silk,  or  linen,  sometimes  in  several  layers,  and  in  some 
cases  inclosed  in  a  spiral  wrapping  of  spring-brass  wire  which 
incloses  each  conductor  of  a  cord  separately,  these  two  spirals 
being  laid  side  by  side  and  braided  over  with  the  familiar  colored 
worsted  braid.  It  is  probably  better,  in  putting  on  the  first  cover- 
ing over  the  tinsel,  to  make  it  a  wrapping  instead  of  a  braid,  as 
the  former  tends  to  bind  the  tinsel  strands  together  more  securely 
than  the  latter,  thus  preventing  any  short  ends  of  the  conductor 
from  piercing  the  covering  and  short-circuiting  the  cord. 

The  question  of  tips  for  receiver  cords  is  one  which  has  received 
much  attention,  as  faulty  tips  are  a  great  source  of  trouble. 
These  are  necessarily  subject  to  rather  rough  usage,  as  it  fre- 
quently happens  that  a  receiver  is  dropped,  thus  allowing  a  heavy 
strain  to  come  on  the  cords,  which  is  usually  most  severe  where 
the  tip  joins  the  cord  proper.  The  connection  shown  in  Fig.  30 
is  one  which  has  become  very  popular. 


THE    TELEPHONE  RECEIVER.  31 

This  is  formed  by  inserting  a  needle  or  pin,  B,  of  No.  14  brass 
wire  tapered  to  a  long  point,  into  the  hollow  of  a  braided  tinsel 
cord  or  spiral  spring,  as  the  case  may  be,  for  one-half  inch,  when 
the  end  is  passed  out  through  the  conductor  and  covering  and 
bent  backward,  forming  a  hook,  as  shown  at  D  and  A,  thus 
combining  the  strength  of  the  conductor  and  covering.  Before 
the  pin  is  put  in,  the  conductor  is  bared  for  a  short  distance  and, 
after  the  pin  is  inserted,  is  wound  with  fine  wire  and  soldered. 
The  tip  is  then  finished  with  a  spiral  of  white  wire  as  shown  at£, 
or  with  a  shell  as  shown  at  C. 

In  order  to  prevent  an   undue  strain  on  the  conductors  when 
receiver  is  dropped,  it  is  best  to  have  the  cord  provided  at  each 


Figs.  31.  and  32. — Supporting  Loop  and  Hook  for  Receiver  Cord. 

end  with  an  auxiliary  loop,  A,  as  in  Fig.  31.  This  loop  is  usually 
a  continuation  of  the  braiding  of  the  cord,  and  may  be  fastened 
to  an  eyelet  in  the  receiver,  or  to  a  small  link  on  one  of  the  bind- 
ing posts.  Another  way  of  accomplishing  this  same  result  is  by 
means  of  a  hook  (Fig.  32)  sewed  to  the  braiding,  just  at  the  fork 
of  the  cord,  which  may  be  closed  by  a  pair  of  pliers  around  a 
screw-eye  or  one  of  the  screws  in  the  binding  post. 


CHAPTER  IV. 

CARBON   TRANSMITTERS. 

MANY  vain  attempts  have  been  made  to  discover  a  satisfactory 
substitute  for  carbon  as  the  variable  resistance  medium  in  tele- 
phone transmitters,  the  patents  on  the  use  of  carbon  electrodes 
having,  until  a  fe\v  years  ago,  formed  one  of  the  mainstays  of 
the  American  Bell  Telephone  Company's  great  monopoly. 

The  theory  of  the  action  of  carbon  in  the  transmitter  has 
been  the  subject  of  much  discussion.  As  previously  pointed  out, 
any  motion  of  the  diaphragm  increasing  the  pressure  between 
the  electrodes  lowers  the  resistance  between  them,  thus  allow- 
ing the  passage  of  a  greater  current.  A  decrease  of  pressure 
produces  the  opposite  result. 

Four  different  explanations  for  this  action  have  been  put  forth, 
and  are  as  follows : 

First,  that  the  electrical  resistance  of  the  carbon  itself  is  caused 
to  vary  by  the  changes  in  pressure. 

Second,  that  a  film  of  air  or  gas  exists  between  the  electrodes, 
and  that  the  thickness  of  this  film  is  varied  by  the  changes  in 
pressure,  thus  varying  the  resistance.  This  theory  is  apparently 
still  adhered  to  by  Mr.  Berliner.* 

Third,  that  the  peculiar  property  possessed  by  carbon  of  lower- 
ing its  resistance  with  increased  temperature  is  in  the  following 
way  accountable  for  the  action,  in  part  at  least:  that  an  increase 
of  current  (due  to  increased  pressure  and  diminished  resistance 
between  the  electrodes)  causes  a  slight  heating  at  the  point  of 
contact ;  that  this  heating  causes  a  still  further  diminution  of 
resistance  with  an  additional  increase  of  current ;  and  that  con- 
versely a  momentary  decrease  of  current  causes  a  decrease  of 
temperature  with  a  corresponding  additional  increase  of  resist- 
ance and  diminution  of  current. 

Fourth,  that  change  in  resistance  is  due  to  the  variation  in  the 
area  of  contact  between  the  electrodes — that  is,  the  variation  in 
the  number  of  molecules  in  actual  contact.  This  change  in  area 
is  perfectly  apparent  in  the  liquid  transmitter  of  Gray,  and  in  the 

*  "  Microphonic  Telephonic  Action,"  by  ]£mile  Berliner,  American  Electrician^ 
March,  1897. 

32 


CARBON   TRANSMITTERS.  33 

case  of  solid  electrodes  may  be  well  illustrated  by  the  following 
well-known  experiment : 

If -a  billiard  ball  be  gently  pressed  on  a  plain  marble  slab 
coated  with  graphite,  the  area  of  contact  of  the  ball  with  the  slab 
will  be  indicated  by  a  small  dot  of  graphite  on  the  ball.  If,  now, 
the  ball  be  dropped  from  a  considerable  height,  it  will  be  noticed 
that  the  spot  of  graphite  on  the  ball  is  much  larger,  showing  that 
the  ball  has  flattened  out  to  a  considerable  extent,  owing  to  the 
greater  pressure  exerted.  This  demonstrates  clearly  the  varia- 
tion in  area  of  contact  between  two  bodies,  due  to  variations  of 
pressure  between  them.  Of  course,  if  the  two  bodies  are  con- 
ductors of  electricity,  the  resistance  between  them  will  vary 
inversely  and  the  current  directly  as  the  area  of  contact. 

It  seems  most  probable  to  the  writer  that  of  the  above  explana- 
tions, the  fourth  is  the  true  one,  and  that  none  of  the  others  aid 
in  any  perceptible  degree  in  producing  desirable  effects  in  the 
microphone. 

As  to  the  first  explanation,  that  the  resistance  of  the  carbon 
itself  changes  under  pressure,  experiments  have  been  made  with 
long  carbon  rods ;  and  with  measuring  instruments  of  ordinary 
sensibility  no  difference  whatever  could  be  detected  in  the  resist- 
ance of  a  rod  when  the  pressure  on  it  was  varied  from  zero  up  to 
the  crushing  point,  care  being  taken  that  all  contacts  in  circuit 
were  not  subjected  to  the  change  in  pressure. 

As  to  the  layer  of  air  theory,  Professor  Fessenden  has  thrown 
some  light  upon  it,*  by  showing  that  if  the  layer  of  air  were  in  the 
ordinary  gaseous  state,  its  resistance  would  be  almost  infinite, 
while  if  it  existed  in  some  peculiar  condensed  state  of  which  we 
know  little,  but  in  which  air  might  be  conceived  to  be  a  conduc- 
tor, then  the  law  of  change  of  resistance  between  the  electrodes 
would  be  different  from  what  it  has  actually  been  found  to  be. 
On  the  other  hand,  the  curves  plotted  with  resistances  as  ordi- 
nates  and  with  distances  as  abscissae  have  been  found  by  Pro- 
fessor Fessenden  and  by  Messrs.  Ross  and  Dougherty  to  exactly 
agree  with  the  form  obtained  from  theoretical  considerations 
on  the  basis  that  the  change  in  resistance  is  due  to  area  of  sur- 
face contact  alone. 

As  to  the  third  explanation,  it  may  be  said  that  the  very  fact 
that  the  increase  of  current  is  needed  to  cause  the  rise  of  tempera- 
ture seems  to  preclude  the  supposition  that  the  rise  of  temperature 
should  cause  the  diminution  of  resistance  with  its  corresponding 

*"  Microphonic  Telephonic  Action,"  by  Professor  R.  A.  Fessenden,  A merican 
Electrician,  May,  1897. 


34 


AMERICAN   TELEPHONE   PRACTICE. 


increase  of  current  in  time  to  do  any  good.  The  heating  effects  in 
carbon  are  comparatively  slow,  and  it  would  seem  that  the  changes 
in  temperature  would  lag  slightly  behind  the  changes  in  current 
producing  them,  in  such  a  manner  as  to  be  detrimental  to  tele- 
phone transmission. 

This  property  of  carbon,  of  lowering  its  resistance  with  in- 
creased temperature,  is,  however,  important  in  that  when  the 
transmitter  becomes  warm  from  constant  use  its  resistance  as  a 
whole  is  decreased.  When  the  transmitter  is  heated  the  total 
resistance  of  the  circuit  is  lowered,  and  the  changes  in  resistance 
produced  by  the  sound  waves  therefore  bear  a  greater  ratio  to 
this  total  resistance  with  corresponding  increase  of  efficiency. 

It  is  certainly  most  fortunate  that  in  one  substance — carbon — 
should  be  found  all  of  the  qualifications  which  make  it  particu- 
larly desirable  for  microphonic  work.  It  produces  the  change  in 
resistance  with  changes  in  surface  contact,  all  things  considered, 
better  than  any  other  known  substance,  possesses  the  desirable 


ig-  33.— The  Blake  Transmitter. 


property  of  lowering   its   resistance  when   heated,  and  is  elastic, 
non-corrosive,  non-fusible,  cheap,  and  easily  worked. 

The  form  of  transmitter  almost  universally  used  in  this  country 
up  to  within  a  few  years  ago,  and  still  largely  used,  is  that  de- 
vised by  Francis  Blake  of  Boston.  This  instrument  is  shown  in 
Fig-  33,  in  which  B  represents  a  metal  ring  or  frame  for  holding 
the  mechanism  of  the  instrument.  It  is  screwed  to  the  cover,  A', 
of  the  box  A,  and  has  two  diametrically  opposed  lugs,  B'  B*.  On 
this  ring  is  mounted  the  diaphragm,  C,  of  rather  heavy  sh~~<- 


CA KBO.V    TRA NSMITTERS. 


35 


iron,  supported  in  a  rubber  ring,  r,  stretched  around  its  edge,  and 
is  held  in  place  by  two  damping  springs,  D  D,  each  bearing 'on  a 
small  block  of  soft  rubber,  a,  resting  on  the  diaphragm  at  a  point 
near  its  center.  The  object  of  these  damping  springs  is  to  pre- 
vent too  great  an  amplitude  of  vibration  of  the  diaphragm,  and 
also  to  keep  it  from  vibrating  in  separate  parts  instead  of  as  a 
unit. 

Opposite  the  center  of  the  diaphragm  is  the  orifice,  £,  in  the 
cover,  A',  so  hollowed  out  as  to  form  a  mouthpiece.  The  adjust- 
ing lever,  F,  is  attached  to  the  spring,/,  secured  to  the  lug,  B\  of 
the  ring,  B.  The  lower  end  of  this  lever  rests  upon  an  adjusting 
screw,  G,  in  the  lug,  B\  which  is  drilled  and  slotted  as  shown 
to  prevent  the  screw  from  working  loose.  On  the  back  of  the  dia- 
phragm and  at  its  center  is  placed  the  front  electrode,  consisting 
of  a  small  bar,  e,  of  platinum  ;  one  end  of  the  bar  rests  against 
the  diaphragm,  while  the  other  end  is  brought  to  a  blunt  point 
and  is  in  contact  with  the  back  electrode,  e.  The  electrode,  e,  is 
supported  independently  upon  a  light  spring,  c,  mounted  on  the 
lever,  Ft  but  insulated  from  it.  This  spring  tends  to  press  away 
from  the  diaphragm  and  toward  the  back  electrode.  The  back 
electrode  is  formed  of  a  block  of  carbon,  e,  set  into  a  brass  block, 
g,  of  considerable  weight,  mounted  on  a  spring,  d,  supported  on 
the  adjusting  lever,  F.  This  spring,  d,  has  a  tension  in  the  oppo- 
site direction  to  that  of  the  spring,  c,  and  being  stronger  than  the 
latter  it  keeps  the  electrode,  e,  in  contact  with  the  diaphragm. 

It  is  seen  that  instead  of  having  one  of  the  electrodes  held  in 
fixed  position  while  the  other  is  pressed  against  it  with  greater  or 
less  force  by  the  vibration  of  the  diaphragm  with  which  it  is  con- 
nected, both  electrodes  are  supported  in  such  manner  as  to  move 
freely  with  the  diaphragm,  but  the  outer  electrode  is  so  weighted 
that  its  inertia  will  offer  enough  resistance  to  the  slight  and  rapid 
vibrations  of  the  diaphragm  to  give  a  varying  pressure  between 
the  electrodes  and  consequent  changes  of  the  resistance  of  the 
circuit.  By  this  means  the  initial  pressure  between  the  two  elec- 
trodes will  not  be  affected  by  changes  of  temperature,  and  the 
adjustment  will  therefore  be  more  nearly  permanent. 

This  transmitter  is  very  delicate,  and  transmits  the  quality  of 
the  voice  in  a  manner  unexcelled  by  others.  It  is,  however,  lack- 
ing in  power,  especially  when  compared  with  instruments  of  later 
design.  Besides  this,  it  has  a  tendency  to  rattle  or  break  contact 
when  acted  on  by  loud  noises. 

Fig.  34  illustrates  the  Crossley  transmitter,  introduced  into 
Europe  early  in  1879.  This  well  illustrates  the  class  very 


AMERICAN    TELEPHONE  PRACTICE. 


appropriately  termed  "multiple-electrode."  Transmitters  de- 
vised by  Johnson,  Govver,  Ader,  D'Arsonval,  Turnbull,  and 
many  others  are  of  this  type,  and  differ  merely  in  the  arrange- 
ment and  number  of  electrodes.  They  give  much  more  powerful 
results  than  the  transmitters  having  a  single  pair  of  electrodes, 
but  most  of  them  are  subject  to  the  grave  defect  of  breaking 
the  circuit  entirely  when  subjected  to  loud  noises. 

In  this  figure,/  represents  a  diaphragm  formed  of  a  thin  piece 
of  pine  board  about  y  thick  and  mounted  on  a   supporting  ring, 


Fig.  34. — The  Crossley  Transmitter. 

K.  Fastened  to  this  diaphragm  are  four  carbon  blocks,  F  G  H 
and  /,  in  the  relative  positions  shown.  These  are  hollowed  out 
to  receive  the  conical  ends  of  the  carbon  pencils,  ABC  and  D, 
which  are  supported  loosely  between  them.  The  blocks,  H  and 
G,  form  the  terminals  of  the  transmitter.  The  current  divides  at 
the  block,  Ht  and  passes  through  the  pencils,  A  and  C,  in  multi- 
ple to  the  blocks,  Fand  7,  and  thence  through  the  pencils,  B  and 
D,  to  the  other  electrode,  G.  Vibrations  of  the  diaphragm  cause 
variations  in  the' intimacy  of  contact  between  the  eight  points  of 


Fig.  35. — The  Turnbull  Transmitter. 

support  of  the  four  rods,  and  thus  produce  the  desired  fluctua- 
tions in  resistance.  It  is  seen  that  this  is  merely  a  modification 
of  the  Hughes  microphone,  the  principles  being  the  same,  but 
the  multiple  contact  allows  a  greater  current  to  pass  through  the 
transmitter,  and  at  the  same  time  produce  greater  changes  in  this 
current  than  in  the  original  form,  where  a  single  pencil  was  used. 
Moreover,  the  liability  of  "  rattling"  is  greatly  reduced. 


CARBON   TRANSMITTERS.  37 

Fig.  35  shows  the  Turnbull  transmitter,  which  has  been  used 
to  a  considerable  extent  in  this  country,  even  until  recently.  In 
this  figure,  A  is  the  diaphragm  of  thin  wood,  on  the  back  of  which 
is  mounted  the  bracket,  B.  Pivoted  on  a  rod,  b,  carried  by  this 
bracket,  are  several  carbon  rods  or  pendants,  a,  which  rest  at 
their  lower  end  against  a  carbon  rod,  c,  carried  on  a  bracket,  C, 
also  mounted  on  the  diaphragm.  The  rods,  b  and  C,  form  the 
terminals  of  the  transmitter,  and  the  current  passes  from  one  of 
them  through  the  carbon  pendants  in  multiple  to  the  other. 
The  variable  resistance  contact  is  mainly  between  the  rod,  C, 
and  the  pendants,  a,  although  by  making  the  rod,  b,  of  carbon 
also  an  additional  effect  is  obtained  between  it  and  the  pendants. 

Fig.  36  shows  still  another  form  of  the  multiple-electrode 
transmitter,  using  carbon  balls  instead  of  pencils  or  pendants. 
A  represents  the  vibratory  diaphragm  of  carbon  ;  B  a  plate  of 
carbon  having  a  number  of  cylindrical  cavities,  t  t,  upon  one 
side.  Fitting  loosely  in  each  cavity  is  a  ball  of  carbon.  The 


Fig.  36. — The  Clamond  Transmitter. 

depth  of  the  cavities  is  a  little  less  than  half  the  diameter  of  the 
balls,  and  the  diaphragm  is  so  placed  in  front  of  the  plate  that 
the  balls,  following  their  tendency  to  roll  out  of  the  cavities,  will 
rest  against  its  inner  surface  and  also  upon  the  edges  of  the  cavi- 
ties. Many  other  forms  of  instruments  have  been  devised  using 
one  or  more  balls  held  in  various  positions  between  carbon  plates. 
Some  are  used  to-day,  but  all  the  transmitters  so  far  described 
are  being  rapidly  replaced  by  the  Runnings  form  of  instrument, 
which,  as  has  already  been  stated,  uses  carbon  "  dust "  or  granules 
for  the  variable  resistance  medium. 

Among  the  earlier  forms  of  the  granular  transmitter  is  a  very 
efficient  one  designed  by  Emile  Berliner,  and  called  the  "  Ber- 
liner Universal."  In  this  the  diaphragm,  D  (Fig.  37),  is  of  car- 
bon, and  is  mounted  horizontally  in  a  case  formed  of  the  two 
pieces,  A  and  B,  of  hard  rubber,  a  brass  ring,  R,  being  clamped 
above  it  to  insure  good  electrical  contact.  Secured  to  the  en- 
larged head,  f,  of  the  screw,  s,  mounted  on  the  block,  B,  is  a 
cylindrical  block  of  carbon,  on  the  lower  face  of  which  are 


3S  AMERICAN   TELEPHONE  PRACTICE. 

turned  several  concentric  V-shaped  grooves.  The  points  formed 
between  these  grooves  almost  touch  the  diaphragm.  The  finely 
divided  carbon,  c,  rests  on  the  diaphragm,  and  is  confined  in  the 
space  between  it  and  the  carbon  block  by  a  felt  ring,  F,  which 


Fig-  37- — -The  Berliner  Universal  Transmitter. 

surrounds  the  latter  and  bears  lightly  against  the  diaphragm. 
To  the  center  of  the  back  plate  a  soft-rubber  tube,  r,  is  fixed 
which  is  of  sufficient  length  to  make  contact  with  the  diaphragm, 
its  function  being  that  of  a  damper  to  the  vibrations  of  the  dia- 
phragm. The  mouthpiece,  M,  is  so  curved  as  to  conduct  the 
sound  waves  against  the  center  of  the  diaphragm.  This  trans- 


ig.  38.  —  Details  of  Solid  Back  Transmitter. 


mitter  was  used  to  a  considerable  extent  by  the  American  Bell 
Telephone  Company,  and  has  now  been  entirely  replaced  for 
long  distance  work  by  the  White  transmitter. 

The    White,  or   "  solid    back,"    transmitter,  as    it  is  called,  is 


CA  RB  ON   TRA  NSM1 T  TERS. 


39 


shown  in  Figs.  38  and  39,  the  latter  giving  a  clear  idea  of  the 
construction  of  the  working  parts  of  the  transmitter,  the  back 
casing  being  removed.  The  upper  portion  of  Fig.  38  shows  the 
section  of  the  complete  instrument.  The  sections  of  Figs.  38 
and  39  are  taken  on  planes  at  right  angles  to  each  other.  The 
separate  parts  of  the  "  resistance  button  "  of  the  instrument  are 
shown  in  the  small  cut  at  the  oottom  of  Fig.  38.  This  instru- 
ment has  proven  remarkably  successful  in  practice,  it  being  able 
to  stand  a  very  heavy  current  without  undue  heating.  Besides 
this,  the  tendency  of  the  granules  to  settle  down  in  a  compact 
mass,  commonly  called  "  packing,"  is  greatly  diminished. 

F  is  of  cast  brass  turned  to  form  the  front  piece  of  the  trans- 
mitter case,  and  is  held,  as  shown,  in  the  hollow  shell,  C,  the  two 
pieces  forming  a  complete  metallic  casing  for  the  working  parts 
of  the  instrument.  The  sound-receiving  diaphragm,  D,  of  alu- 


ig.  39.— Sectional  View  Solid  Back  Transmitter. 


minum,  is  encased  in  a  soft-rubber  ring,  <?,  held  in  place  by  two 
damping  springs,//,  as  in  the  Blake  transmitter.  Wis  a  heavy 
metallic  block  hollowed  out,  as  shown,  to  form  'a  casing  for  the 
electrodes.  The  inner  circumferential  walls  of  this  block  are 
lined  with  a  strip  of  paper,  /.  This  block  is  mounted,  as  shown, 
on  a  supporting  bracket,  P,  secured  at  its  ends  to  the  front  cast- 
ing, F.  The  back  electrode,  B,  of  carbon  is  secured  to  the  face 
of  the  metallic  piece,  a,  which  is  screw-threaded  into  the  block, 
W.  E  is  the  front  electrode,  also  of  carbon,  carried  on  the  face 


40  AMERICAN   TELEPHONE  PRACTICE. 

of  the  metallic  piece,  b.  On  the  enlarged  screw-threaded  por- 
tion, /,  of  the  piece,  b,  is  slipped  a  mica  washer,  M,  held  in 
place  by  the  nut,  u.  This  washer  is  of  sufficient  diameter  to 
completely  cover  the  cavity  in  the  block,  W,  when  the  electrode 
is  in  place.  After  the  required  amount  of  granular  carbon  has 
been  put  into  the  cavity,  and  the  front  electrode  put  in  position, 
the  cap,  c,  is  screwed  in  its  place  on  the  block,  W,  as  shown,  and 
binds  the  mica  washer,  m,  firmly  against  the  face  of  the  block,  B, 
thus  confining  the  granules  in  their  place.  The  electrodes  are  of 
somewhat  less  diameter  than  the  paper-lined  interior  of  the 
block,  W,  so  that  there  is  a  considerable  space  around  the 
periphery  of  the  former,  which  is  filled  with  carbon  granules. 
This  prevents  the  binding  of  the  free  electrode  against  the  edge 
of  its  containing  chamber,  and  also  allows  room  for  the  granules 
directly  between  the  electrodes  to  expand  when  heated  by  the 
passage  of  current.  The  screw-threaded  portion,/',  of  the  piece, 
b,  passes  through  a  hole  in  the  center  of  the  diaphragm,  and  is 
clamped  firmly  in  place  by  the  nuts,  /  /'.  M  is  the  mouthpiece 
of  hard  rubber,  screw-threaded  in  an  opening  in  the  front  block, 
F.  Any  vibration  of  the  diaphragm  is  transmitted  directly  to 
the  front  electrode,  E,  which  is  allowed  to  vibrate  by  the  elas- 
ticity of  the  mica  washer,  m.  The  back  electrode  is,  of  course 
stationary,  being  firmly  held  by  the  bracket,  P. 

The  back  electrode  is  in  metallic  connection  with  the  frame  of 
the  instrument,  which  forms  one  terminal.  The  other  terminal, 
T,  is  mounted  on  an  insulating  block,  /,  and  is  connected  by  a 
flexible  wire  with  the  front  electrode,  E.  This  construction  is 
best  shown  in  Fig.  39. 

This  transmitter  is  now  used  on  all  of  the  long-distance  lines  of 
the  Bell  Company,  and  has  given  excellent  service.  It  was 
formerly  always  used  with  three  Fuller  cells  in  series,  but  the 
tendency  is  now  to  use  but  two. 

The  following  data  concerning  the  dimensions  and  material 
used  in  this  instrument  will,  it  is  believed,  be  found  of  much 
interest : 

Diaphragm — aluminum,  2\"  diameter  and  .022*  thick. 

Rubber  band  or  gasket — f"  wide,  2-f"  dpuble  length,  very 
soft  and  elastic. 

Front  electrode — carbon,  hard  and  polished,  f  J-"  diameter,  Ty 
thick. 

Back  electrode — carbon,  hard  and  polished,  -fj"  diameter,  TV" 
thick. 

Mica  diaphragm — f  J"   diameter,  very  thin. 


CARBON    TRANSMITTERS. 


Back  electrode  chamber — inside  diameter,  j-*,  depth  J^",  clear- 
ance between  sides  of  electrode  and  walls  of  chamber  ^2". 

Distance  between  electrodes  about  .04*. 

Damping  spring — spring  steel,  J|-"  wide,  .010"  thick,  iTy  long  ; 
bent  at  right  angles  when  not  in  place.  The  one  which  rests 
near  center  of  diaphragm  is  tipped  with  soft  rubber  and  also  with 
felt ;  the  outer  spring,  with  rubber  only. 

Fig.  40  illustrates  the  Colvin  transmitter.  Although  this  is  an 
efficient  instrument  and  extremely  unique  in  design,  it  is  very 
little  used.  The  shell  is  formed  of  two  pieces,  A,  provided  with 
the  usual  mouthpiece,  and  B,  fitting  into  a  recess  in  the  piece,  A. 
The  space  in  which  the  diaphragm  fits  is  made  large  enough 
to  hold  the  diaphragm  very  loosely  so  that  it  and  the  cell  it 
carries  may  vibrate  with  great  freedom  under  the  impact  of 
sound  waves.  Upon  the  diaphragm  is  supported  a  hollow  cylin- 
drical cell,  D,  of  insulating  material  (shown  in  the  small  cut  at  the 


Fig.  40. — The  Colvin  Transmitter. 

left),  carrying  two  metallic  electrodes,  E  E ,  insulated  'from  each 
other.  To  these  electrodes  are  connected  the  circuit  terminals, 
G  G.  The  shell,  D,  is  clamped  firmly  to  the  diaphragm,  C,  by  a 
bolt,  F,  thus  closing  the  chamber  containing  the  granules.  To 
prevent  the  access  of  moisture  to  the  carbon  granules  the  joint 
between  the  diaphragm  and  the  edge  of  the  shell  is  hermetically 
sealed.  The  diaphragm  is  of  aluminum,  and  being  loosely 
mounted  is  free  to  vibrate  with  great  amplitude.  One  of  the 
striking  features  of  this  instrument  is  that  the  two  electrodes, 
E  E,  are  fixed  with  relation  to  each  other,  the  variation  in  resist- 
ance being  obtained  by  the  variation  in  pressure  between  the 
electrodes  and  the  carbon  granules,  due  to  the  inertia  of  the 
latter,  and  also  to  the  shaking  up  of  the  granules  themselves,  and 
the  consequent  variation  of  their  intimacy  of  contact  with  each 
other. 

Fig.  41  shows  the  Button  transmitter  now  manufactured  by 


AMERICAN    TELEPHONE  PRACTICE. 


the  Phoenix  Interior  Telephone  Company.  The  variable  resist- 
ance parts  comprise  a  pair  of  carbon  buttons,  F  and  G,  each 
surrounded  by  a  sleeve  of  cloth,  //  and  /,  the  abutting  edges,  h 
and  *,  of  which  are  frayed  out  so  as  to  form  an  intimate  but 
yielding  contact.  These  not  only  serve  to  damp  the  vibrations 
of  the  diaphragm,  but  form  with  the  buttons,  F  and  G,  a  closed 
chamber  in  which  the  granular  carbon  is  placed.  The  button,  Ft 
is  secured  to  the  diaphragm,  K,  as  shown,  while  the  button,  G,  is 


Fig.  41. — The  Button  Transmitter. 

rigidly  secured  to  the  case  of  the  instrument,  and  is  insulated 
therefrom.  The  wire,  O,  leading  from  the  bolt,  L,  which  secures 
the  button,  G,  in  place,  forms  one  terminal  of  the  instrument,  the 
casing  itself  the  other. 

The  Ericsson  transmitter,  manufactured  in  Sweden,  is  being 
imported  into  this  country  to  a  considerable  extent  as  a  com- 
panion piece  to  the  Ericsson  receiver.  This  transmitter  gives 
a  very  clear,  soft  tone,  and  requires  little  battery  power.  On  the 


Fig.  42. — The  Ericsson  Transmitter. 

whole  it  is  a  very  efficient  instrument.  It  is  shown  in  section  in 
Fig.  42,  in  which  a  is  the  sound-receiving  diaphragm  held  against 
a  shoulder  in  the  brass  casing,  c,  by  two  thin  leaf-springs,  not 
shown,  each  spring  having  two  branches,  so  as  to  give  in  all  four 
points  bearing  on  the  diaphragm. 


CARBON   TRANSMITTERS. 


43 


For  preventing  moisture,  especially  the  moisture  contained  in 
the  breath,  from  entering  beyond  the  diaphragm  in  the  casing  in 
which  the  diaphragm  and  other  parts  of  the  microphone  are 
situated,  a  thin  disk,  b,  of  silk  impregnated  with  lacquer  is  placed 
in  front  of  the  diaphragm,  a,  between  it  and  the  mouthpiece. 
The  border  of  the  disk  b,  as  well  as  that  of  the  diaphragm,  a,  is 
close  to  the  wall  of  the  casiqg,  c.  { 

The  metal  plate,  d,  mounted  onvthe  rear  side  of  the  diaphragm 
forms  the  front  electrode,  and  for  that  purpose  is  gold-plated. 
The  backwardly  bent  rim  of  the  plate,  d,  surrounds  the  fore  part 
of  a  soft  ring,  e,  on  the  carbon  block,/,  and  serves  to  prevent  the 
carbon  grains  from  falling  out  of  the  chamber.  This  soft  ring  is 
made  of  raveled  felt,  and  does  not  impede  the  movements  of  the 


Fig.  43. — Western  Telephone  Construction  Co.'s  Transmitter. 

diaphragm  further  than  to  prevent  their  amplitude  from  becom- 
ing too  great  for  good  transmission. 

The  diaphragm  is  further  damped  by  the  coiled  spring  resting 
in  a  chamber  in  the  center  of  the  carbon  electrode.  This  spring 
rests  on  a  tuft  of  cotton  or  felt,  which  in  turn  bears  on  the  center 
of  the  front  electrode. 

The  transmitter  of  the  Western  Telephone  Construction  Com- 
pany (Fig.  43)  is  probably  the  simplest  manufactured.  The  whole 
front  case,  A,  of  the  transmitter  is  of  a  turned  brass  casting.  It 
is  shouldered  inside  to  form  a  seat  for  the  diaphragm,  D,  and 
threaded  to  engage  an  insulating  cup,  B,  carrying  the  back  elec- 
trode, C.  This  cup  is  screwed  directly  on  a  flange  of  the  rocker 
arm,  E,  from  the  inside.  The  central  screw  which  holds  the  back 
electrode  in  place  also  passes  into  the  iron  rocker  arm,  thereby 
making  it  one  terminal  of  the  transmitter.  The  back  electrode,  C, 


44  AMERICAN    TELEPHONE   PRACTICE. 

is  large,  being  i|"  in  diameter  and  f"  thick.  The  chamber  in 
which  this  block  is  mounted  allows  about  |"  space  all  around  the 
electrode,  which  space,  as  well  as  that  between  the  diaphragm 
and  the  back  electrode,  contains  granular  carbon.  The  diaphragm, 
D,  is  of  carbon,  usually  .016"  thick  and  2Ty  in  diameter,  the  free 
portion  being  i-J-f"  in  diameter.  The  distance  between  the  back 
electrode  and  the  diaphragm  is  fa".  If  the  entire  space  in  the 
chamber  were  filled  with  granules,  the  whole  rear  surface  of  the 
diaphragm  would  serve  as  a  front  electrode.  The  space,  however, 
is  only  half  filled,  and  only  the  lower  half  therefore  is  actively 
engaged  as  an  electrode. 

It  would  seem  from  theoretical  considerations  that  better  re- 
sults would  be   obtained  by   using  only  the   center    of  a  carbon 


Fig.  44. — American  Electric  Transmitter. 

diaphragm  as  the  electrode  and  covering  up  all  other  portions. 
Many  companies  are  following  this  idea  ;  but  a  careful  series  of 
experiments  have  failed  to  show  this  to  be  true.  The  trans- 
mitter described  above  is  powerful  and  articulates  well. 

Fig.  44  is  chiefly  interesting  as  illustrating  a  form  of  rocker 
arm  made  by  the  American  Electric  Telephone  Co.  This  arm 
not  only  enables  the  transmitter  to  be  adjusted  as  to  height, 
but  also  laterally  so  that  it  may  be  pushed  back  out  of  the  way. 

The  transmitter  manufactured  by  this  company  and  shown 
in  this  cut  resembles  the  "  solid  back  "  in  external  appearance. 
It  has  a  diaphragm  of  thin  iron  to  which  a  thin  carbon  electrode 
is  riveted.  The  back  electrode  is  mounted  in  an  insulating  block 
and  surrounded  with  a  layer  of  cloth,  which  rests  lightly  against 
the  diaphragm  and  holds  the  granules  in  place. 


CARBON    TRANSMITTERS. 


45 


.In  granular-carbon  transmitters  much  trouble  has  been  ex- 
perienced from  what  is  commonly  known  as  "  packing."  This  is 
sometimes  caused  by  the  granules  settling  into  a  compact  mass 
by  the  constant  agitation  due  to  the  sound  waves.  As  a  natural 
result  the  granules  arrange  themselves  in  layers  according  to 
their  size,  the  small  ones  working  toward  the  bottom.  In  this 
state  the  entire  mass  becomes  very  compact,  thus  losing  the  ad- 
vantages of  loose  contact  and  impairing  the  transmitting  qualities 
of  the  instrument. 

Sometimes  packing  is  caused  by  a  few  granules  becoming 
wedged  between  the  diaphragm  and  the  back  electrode,  thus 
preventing  the  free  vibration  of  the  diaphragm.  Nearly  all 
transmitters  may  be  packed  by  pressing  the  lips  firmly  against 
the  mouthpiece  and*  sucking  in  the  breath.  The  diaphragm  is 
thus  strained  away  from  the  back  electrode  and  the  granules 
settle  into  the  space  so  formed.  When  the  diaphragm  is  re- 


Fig.  45.— Transmitter  with  Mouthpiece  Agitator. 

leased  it  binds  tightly  against  the  granules,  and  the  transmitter  is 
thus  rendered  perfectly  "dead."  A  sharp  rap  from  beneath 
will  often  restore  it  to  its  former  efficiency. 

Another  cause  of  packing  is  moisture  from  the  breath  of  users 
of  the  telephone,  which  soaks  through  or  around  the  diaphragm 
and  destroys  the  "  life  "  of  the  electrodes.  For  this  reason  her- 
metically sealed  transmitters  are  desirable. 

Fig.  45  shows  a  simple  contrivance  for  preventing  packing.  A 
represents  the  front  of  the  transmitter  box  and  B  the  brass  shell 
containing  the  working  parts  of  the  transmitter.  A  cylindrical 
portion  of  the  shell  extends  through  the  front  board,  A,  and 
carries  the  mouthpiece,  M.  C  is  the  hard-rubber  back  plate  of 
the  transmitter.  The  spring,  E,  presses  against  the  screw,  K, 
projecting  from  the  center  of  the  back  plate,  C,  and  forms  one 


46  AMERICAN    TELEPHONE  PRACTICE. 

terminal;  the  spring,  D,  having  two  arms,  d d,  bearing  against 
the  casing,  B,  forming  the  other.  By  manually  turning  the 
mouthpiece  the  entire  casing  of  the  transmitter  and  the  parts 
contained  therein  may  be  rotated,  and  the  granular  carbon,  always 
falling  to  the  bottom  of  its  containing  chamber,  is  effectively 
stirred  up.  The  arms,  d  d,  of  the  spring,  D,  make  a  sliding  con- 
tact on  the  casing,  B,  while  the  screw,  K,  turns  pivotally  under 
the  spring,  E.  This  arrangement  effectively  remedies  the  pack- 
ing difficulty,  but  much  trouble  is  often  caused  by  poor  contacts 
between  the  springs  and  the  parts  of  the  transmitter  on  which 
they  rest. 

Means  have  also  been  devised  for  automatically  turning  the 
transmitter  or  otherwise  agitating  the  carbon  by  the  removal  of 
the  receiver  from  the  hook.  These,  however,  have  not  generally 
been  found  desirable. 

In  Fig.  46  is  shown  a  transmitter  recently  designed  by  Mr.  T. 
F.  Ahearn,  which  is  interesting  as  showing  one  of  the  many 


Fig.  46. — Ahearn  Transmitter. 

attempts  to  produce  changes  in  area  of  contact  without  changes 
of  pressure. 

E  is  a  carbon  electrode  attached  to  the  center  of  the  metal  dia- 
phragm, A,  and  forms  the  terminal  electrode  to  which  the  wire,  D, 
is  attached.  This  electrode  consists  of  a  plate  or  plates,  of  either 
semicircular  or  triangular  form,  as  shown. 

The  back  electrode,  G,  is  of  similar  form  and  is  carried  on  the 
spring,/,  in  such  manner  as  to  overlap  and  rest  on  the  front  elec- 
trode, E.  The  pressure  between  the  two  may  be  regulated  by  the 
thumb-screw,  as  shown. 

It  is  claimed  that  in  this  no  variation  in  pressure  can  be  caused 
by  the  vibration  of  the  diaphragm,  but  that  the  electrodes  simply 
slide  over  each  other,  the  shape  of  the  surfaces  in  contact  ampli- 
fying the  changes  in  contact  area. 

Fig.  47  shows  one  of  the  attempts  to  increase  the  efficiency  of 
the  microphone,  but  results  so  far  obtained  from  this  and  similar 


CARBON    TRANSMITTERS. 


47 


experiments  have  not  proved  of  sufficient  value  to  warrant  the 
additional  complexity  of  parts.  This  instrument  consists  of  a 
double  Blake  transmitter,  with  a  pair  of  electrodes  on  each  side  of 
the  diaphragm.  The  action  of  the  electrodes,  e  and  t,  is  the  same 
as  that  of  the  electrodes  of  the  regular  Blake  instrument.  The 
electrodes,  <af  and  /z,  however,  being  on  the  side  of  the  diaphragm 
toward  the  speaker,  serve  also  to  vary  the  resistance  of  their 
point  of  contact,  but  an  increase  in  resistance  between  e  and  i  is 
accompanied  by  a  decrease  in  resistance  between  d  and  //,  and 
vice  versa.  The  induction  coil  used  with  this  instrument  has  two 
oppositely  wound  primary  coils,  M  and  N.  The  coil,  M,  is  in 
circuit  with  the  pair  of  contacts,  d  and  h,  while  the  coil,  N,  is  in 
circuit  with  the  contacts,  e  and  i.  As  these  coils  are  wound  in 
opposite  directions,  and  as  an  increase  of  current  flowing  from 
the  battery,  B,  through  one  of  them  is  always  accompanied  by  a 
decrease  of  current  through  the  other,  it  follows  that  their  induct- 
ive effects  on  the  secondary  coil,  5,  will  be  added. 

A  transmitter  constructed  with  the   idea  of  producing  actual 


Fig.  47. — Double  Transmitter. 

alternations  in  the  current  flowing  in  the  primary  has  recently 
been  patented  by  Messrs.  G.  F.  Payne  and  Wm.  D.  Gharky  of 
Philadelphia.  It  is  of  unique  design  and  produces  very  power- 
ful results.  It  is  designed  to  operate  on  the  principle  of  a  pole- 
changing  switch,  and  in  Fig.  48  its  analogy  to  that  familiar  form 
of  circuit-changing  device  is  shown.  The  cuts  in  the  upper  por- 
tion of  this  figure  show  the  two  positions  of  a  pole-changer,  and 
it  will  be  evident  that  the  direction  of  current  through  the  coil,  W, 
will  depend  on  the  positions  of  the  switches  as  shown  by  the 
arrows.  In  the  lower  portion  of  this  figure  the  circuits  and  elec- 
trodes of  the  transmitter  are  diagrammatically  shown.  The  elec- 
trodes, K  G  and  /,  are  stationary,  while  electrodes,  M  and  N,  move 
in  accordance  with  the  vibrations  of  the  diaphragm,  being  con- 
nected thereto  by  a  piston-rod.  As  the  movable  electrodes  are 


48  AMERICAN   TELEPHONE  PRACTICE. 

by  the  action  of  the  diaphragm  impelled  toward  the  left  the  resist- 
ance to  the  passage  of  a  current  between  the  electrodes,  N  and  G 
and  M  and  K,  is  diminished,  while  it  is  increased  as  between  the 
electrodes,  TV  and  J  and  J/and  G.  Consequently  the  greater  part 
of  the  battery  current  will  pass  to  and  through  the  movable 
electrode,  Ny  the  stationary  electrode,  G,  upward  through  the 
wire  connection,  V,  thence  through  the  wire  connection,  U,  to  the 
stationary  electrode,  AT,  and  thence  through  the  movable  electrode, 
M,  and  to  the  battery.  An  impulse  to  the  right  brings 
the  movable  electrodes  into  the  position  shown  at  the  right-hand 
lower  cut,  and  reverses  the  conditions,  producing,  as  there  shown, 
a  downward  current  through  the  wire,  F,  the  changes  from  the 
one  condition  to  the  other  being  of  course  gradual  and  without 
sensible  interruption,  and  the  result  being  that  the  greater  part  of 


Li  \ 

M'I'M 


Fig.  48.— Diagram  of  Payne  £  Gharky  Transmitter. 

the  battery  current  is  sent  through  the  coil,  W,  first  in  one  direc- 
tion and  then  in  the  other,  following  of  course  the  movement  of 
the  diaphragm. 

The  construction  of  this  transmitter  is  shown  in  Fig.  49,  the 
lower  portion  of  which  shows  an  enlarged  view  of  the  electrodes. 
Parts  B  and  C  form  the  framework  of  the  instrument,  supporting 
the  diaphragm  and  all  working  parts.  D  is  a  cylindrical  box,  in 
which  the  electrodes  are  situated,  carried  on  the  bracket,  C. 
G  is  the  central  stationary  electrode,  constructed  of  brass 
with  carbon  faces  and  connected  to  one  terminal  of  the  primary 
coil,  W.  The  outer  stationary  electrodes  are  each  formed  with 
stems,  as  indicated  at /and  K,  which  extend  through  the  heads, 
F  Fj  off  the  box,  and  are  secured  in  proper  position  by  the  set- 
screws,//.  At  the  inner  end  of  each  rod  is  a  brass  disk  (indi- 


CARBON    TRANSMITTERS. 


49 


cated  at  /'  and  K'},  /'  and  K\  indicating  brass  disks  screwing 
on  the  stems,  J  and  K,  and  acting  to  clamp  a  light  felt  washer,  L, 
between  themselves  and  the  disks,  J',  and  K.  The  disks,/'  K',  are 
each  provided  with  a  carbon  facing,  H.  The  movable  electrodes, 
of  which  one  is  situated  on  each  side  of  the  central  stationary 
electrode,  are  made  up,  as  sho(\vn,  of  two  brass  disks,  such  as  are 
indicated  at  M  M  and  N^N,  a  light  felt  washer,  L,  being  clamped 
between  the  disks  in  each  case.  The  electrode,  N,  is  secured  to  the 
diaphragm  by  a  light  metal  rod,  O,  one  end,  (7*,  of  which  is  shown 
threaded  and  screwing  into  the  electrode  end,  while  the  other 
end,  O',  is  threaded  and  screws  into  the  nut,  A'.  This  rod  is 


Fig.  49. — Details  of  Payne  &  Gharky  Transmitter. 

covered  by  a  non-conducting  jacket,  P,  which  in  turn  is  partly 
inclosed  by  a  conducting-tube,  Q,  which  connects  with  the  elec- 
trode, M.  The  two  electrodes,  Mand  N,  are  thus  rigidly  bound  to 
each  other  and  to  the  diaphragm,  but  are  insulated  from  each 
other.  The  battery  is  connected  between  them  as  shown. 

Granular  carbon  is  placed  between  each  opposite  pair  of  elec- 
trode-faces, there  being  thus  four  separate  bodies  of  granular  car- 
bon, which  are  prevented  from  coming  into  contact  with  each 
other  by  the  light  felt  washers,  L. 

Whether  or  not  the  effects  produced  by  the  action  of  these 
electrodes  will  be  of  great  enough  gain  to  overcome  the  objec- 
tionable complication,  time  must  show;  the  results  obtained, 
however,  are  remarkable. 


5° 


AMERICAN    TELEPHONE  PRACTICE. 


Figs.  50  and  51  show  an  oddity  in  the  form  of  a  granular 
carbon  transmitter  devised  by  Mr.  W.  W.  Jacques  of  the  Bell  Com- 
pany. In  the  ordinary  transmitter  too  much  battery  power  is  to  be 
guarded  against,  as  it  throws  the  electrodes  into  vibration  and 
causes  the  well-known  squealing  or  sizzling  sound.  Mr.  Jacques 
claims  that  when  a  multitude  of  electrodes  in  loose  contact  are 
normally  kept  in  a  state  of  rapid  and  continuous  vibration  by 
such  a  strong  current,  they  are  much  more  sensitive  to  sound 
waves  falling  upon  them  than  they  are  when  at  rest.  He  gives 
as  a  probable  explanation  of  this  that  the  resultant  normal  pres- 
sure existing  between  the  various  pairs  of  electrodes  is  less  when 
all  of  the  electrodes  are  in  vibration  to  and  from  each  other  than 


Figs.  50.  and  51. — Jacques  Transmitter. 

» 

when  they  are  at  rest  ;  and  it  is  well  known  that,  within  certain 
limits,  the  sensitiveness  of  any  microphone  contact  increases  as 
the  normal  pressure  is  decreased. 

He  uses  a  current  at  a  pressure  of  about  20  volts,  which  of 
course  sets  up  a  vibration  of  the  granules,  thereby  maintaining 
them  in  the  "  desired  condition  of  sensitiveness  to  sound  waves." 
The  use  of  such  great  battery  power  also  allows  the  variation  of 
a  greater  current  than  where  the  usual  low  voltage  is  used.  He 
proposes  to  use  this  only  on  long  lines  and  claims  that  "  the 
undulations  of  current  due  to  the  vibrations  of  the  electrodes  of 
the  transmitter  produced  by  the  normal  action  of  the  battery 
will  fade  out  and  disappear  at  a  greater  or  less  distance  from  the 
transmitter  ;  while  the  undulations  of  current  due  to  the  action 


CARBON    TRANSMITTERS.  51 

of  sound  waves  upon  the  normally  vibrating  electrodes  will  per- 
sist, and  the  sounds  be  heard  in  the  telephone  at  the  distant  end 
of  the  line." 

In  order  to  stand  such  a  heavy  current  the  transmitter  is  made 
practically  fireproof  by  the  construction  shown  in  Figs.  50 
and  51,  in  which  A  is  a  cup-shaped  metallic  frame  supporting  the 
operative  parts  of  the  instrument.  C  is  a  metallic  cover  secured 
to  the  frame,  A,  by  screws,  s,  clamping  the  diaphragm,  D,  in  the 
ordinary  manner. 

G  is  a  cylinder  made  of  slate,  having  a  flange,/,  and  secured  to 
the  interior  of  the  cup-shaped  frame,  A,  by  a  brass  ring,  r,  which 
rests  upon  the  flange  and  is  there  held  by  screws, /,  screwing  into 
the  frame. 

E  is  the  back  electrode,  being  a  disk  of  hard  carbon  brazed  to 
a  brass  disk,  K,  a  projection  from  which  lies,  as  shown,  within  a 
hollow  projection,/,  from  frame,  A,  the  two  said  projections  being 
insulated  from  each  other  by  a  cup-shaped  washer  of  vulcanized 
fiber.  P  is  the  front  or  working  electrode,  being  a  disk  of  hard 
carbon  rigidly  secured  to  the  diaphragm,  D,  by  a  screw  and  nut. 
The  two  electrodes,  E  and  P,  fit  the  cylinder,  G,  closely.  For  a 
variable  resistance  material  between  them  granulated  carbon  is 
employed,  the  grains  being  of  such  size  that  they  will  not  pass 
between  the  peripheries  of  the  electrodes,  E  and  P,  and  the  inner 
wall  of  the  cylinder,  G.  These  granules  are  kept  in  violent  vibra- 
tion by  the  strength  of  the  battery  current,  while  serving  also  as 
the  variable  resistance  medium  between  the  working  electrode,  P, 
and  the  back  electrode,  E,  to  take  up  the  vibrations  due  to  vocal 
waves  in  the  ordinary  manner. 

The  back  electrode,  E,  is  made  adjustable  by  means  of  a  spring, 
a,  tending  to  push  the  brass  disk,  K,  into  the  cylinder,  G,  and  a 
brass  thumb-screw,  b.  A  flanged  washer  of  vulcanized  fiber 
insulates,  frame,  A,  from  the  thumb-screw,  b.  The  frame,  A,  is  in 
metallic  connection  with  the  working  electrode,  P,  while  the 
thumb-screw,  b,  is  in  metallic  connection  with  the  back  electrode, 
E.  The  noise  in  the  receiving  instrument,  resulting  from  the 
two  sets  of  vibrations  in  the  transmitting  instrument  at  the  same 
end  of  the  line,  is  not  only  painful  to  the  ear,  but  interferes 
with  the  proper  reception  by  the  ear  of  sounds  coming  from  the 
other  end  of  the  line.  To  obviate  this  difficulty,  the  receiving 
telephone  is  so  constructed  that  the  current  coming  from  the 
transmitting  telephone  at  the  same  end  of  the  line  is  divided 
and  passes  around  the  core  of  the  receiving  instrument  in  two 
directions,  while  the  current  from  the  transmitting  telephone  at 


4- 


52 


AMERICAN    TELEPHONE  PRACTICE. 


the  farther  end  of  the  line  passes  around  the  core  always  in  one 
direction.  The  direction  of  the  windings  is  such  that  in  the 
former  case  the  two  windings  neutralize  each  other  and  produce 


Fig.  52. — Carbon  Electrodes  for  Transmitter. 

no  noise,  while  in  the  latter  the  effects  of  the  two  coils  are  added, 
thus  giving  the  receiver  its  full  power. 

Fig.  52  is  interesting  as  showing  some  of  the  standard  trans- 
mitter electrodes  and  diaphragms  used  in  this  country.  The 
cuts  of  these  were  loaned  by  Mr.  M.  M.  Hayden  of  the  Globe 
Carbon  Co. 


CHAPTER  V. 

i 

INDUCTION   COILS. 

IT  has  already  been  pointed  out  in  the  chapter  on  the  History 
and  Principles  of  the  Battery  Transmitter  that  the  use  of  the  in- 
duction coil  is  of  decided  advantage  in  that  it  allows  the 
changes  in  the  transmitter  to  bear  a  much  larger  ratio  to  the 
total  resistance  of  the  circuit  in  which  these  changes  occur  than 
would  otherwise  be  the  case ;  and  further,  that  by  virtue  of  the 
transformation  from  a  comparatively  low  to  a  high  voltage,  the 
currents  are  much  better  adapted  for  traversing  long  lines  and 
higher  resistances.  It  may  be  further  pointed  out  that  with  the 
same  battery  power  the  current  in  the  primary  circuit  is  much 
greater,  owing  to  the  lower  resistance,  than  if  the  battery  were 
placed  in  the  line  circuit,  and  therefore  the  transmitter  is  not 
only  able  to  produce  a  greater  relative  change  in  the  current 
flowing,  but  to  cause  these  changes  to  act  on  a  larger  current. 

It  should  be  remembered  that  the  current  in  the  primary  cir- 
cuit is  an  undulating  one  and  is  always  in  the  same  direction. 
The  current  in  the  secondary,  however,  is  alternating  in  character, 
changing  its  direction  completely  with  every  large  fluctuation  in 
the  primary  current.  This  latter  feature  is  also  productive  of 
better  results  than  would  be  the  case  were  the  current  in  the  line 
wire  of  an  undulatory  character. 

The  size  and  quality  of  the  iron  core,  the  relation  between  the 
number  of  turns  in  the  primary  and  secondary  windings,  and 
the  mechanical  construction  of  the  induction  coil  are  matters  of  the 
greatest  importance,  and  have  not  in  general  received  the  atten- 
tion which  they  merit.  A  number  of  attempts  have  been  made 
to  calculate  mathematically  the  best  dimensions  and  resistances 
of  the  telephone  induction  coil,  but  the  matter  is  of  such  an 
extremely  complex  nature,  and  all  of  the  quantities  are  subject  to 
such  complex  and  indeterminate  variations,  that  the  results  so  far 
produced  have  been  in  general  unreliable. 

Only  a  few  series  of  experiments  are  on  record  from  reliable 
sources  giving  the  results  of  comparative  tests  between  induc- 
tion coils  of  various  dimensions.  It  may  be  said  that  definite 

53 


54  AMERICAN    TELEPHONE   PRACTICE. 

results  from  any  such  series  of  tests  are  extremely  hard  to  get,  as 
the  quality  and  loudness  of  transmission  is  subject  to  a  very  great 
personal  error,  even  in  the  case  of  experienced  experimenters. 
In  making  comparative  tests  of  any  telephone  apparatus  it 
is  of  the  greatest  importance  that  all  possibility  of  prejudice  on 
the  part  of  the  experimenter  be  removed,  and  in  order  to  do  this 
it  is  essential  that  he  be  in  ignorance  at  all  times  of  the  particular 
instrument  that  he  is  testing.  To  illustrate  this  point,  suppose 
that  three  transmitters  are  being  compared  with  respect  to 
determining  the  general  talking  qualities  of  each.  If  the  party 
at  the  receiving  telephone,  who  is  to  judge  of  the  hearing,  desires 
that  one  of  these  instruments  produces  better  results  than  the 
others,  he  is  very  sure,  even  though  he  be  strictly  honest  with 
himself,  to  conclude  at  the  end  that  that  transmitter  is  by  far  the 
best ;  unless,  of  course,  there  is  a  very  marked  difference  between 
them.  For  this  reason  he  should  be  kept  in  ignorance  of  the 
particular  transmitter  in  circuit  with  the  line  at  any  time,  and 
should  only  be  told  when  changes  are  made.  It  is  well  to  have 
the  instruments  numbered,  the  party  judging  of  the  merits  to 
be  in  ignorance  of  the  transmitters  to  which  those  numbers 
refer. 

It  is  also  a  somewhat  difficult  matter  in  comparing  the  clear- 
ness with  which  instruments  transmit  or  reproduce  speech  to 
select  proper  subject  matter  to  be  transmitted.  It  is  unfair  to 
the  first  instrument  tested  to  repeat  the  same  sentence  or  read 
the  same  matter  in  each  case,  for  the  reason  that  the  listening 
party  becomes  more  or  less  familiar  with  the  matter  to  which  he 
is  listening,  and  therefore  often  catches  words  at  the  second  or 
third  reading  which  he  fails  to  grasp  at  the  first.  It  is  therefore 
better  to  read  a  selection  from  a  certain  article  in  the  first  test, 
and  a  continuation  of  the  same  matter  in  each  successive  test,  so 
that  the  character  of  the  matter  read  will  be  approximately  the 
same  in  each  case.  However,  where  several  instruments  are 
apparently  of  almost  the  same  merit,  and  where  the  transmission 
is  so  good  that  all  of  the  matter  may  be  generally  understood,  it 
has  been  found  that  a  better  way  is  to  prearrange  a  number  of 
series  of  words,  a  different  series  to  be  read  into  each  instrument 
under  test.  Care  must  be  taken,  in  the  selection  of  these  words, 
that  each  series  contains  words  of  the  same  character. 

To  illustrate :  suppose  five  instruments  are  to  be  tested.  Five 
different  series  of  words  may  be  prepared,  containing  such  words 
as  the  following : 


INDUCTION  COILS.  55 

1st  2d  3d  4th  5th 

sign  rhyme  dine  fine  mine 

going  rowing  sewing  mowing          throwing 

missile  thistle  whistle  bristle  gristle 

D  EG  P  B 

etc.  etc.  etc.  etc.  etc. 

( 

Each  list  should  contain  about  forty  words;  and  as  the  words  in 
each  series  will  be  seen  to  differ  from  the  corresponding  words  in 
the  others  to  only  a  slight  extent,  no  instrument  can  be  said  to 
have  an  easier  list  than  the  others.  The  first  list  should  be  read 
into  the  first  instrument  slowly  enough  for  the  receiving  party  to 
write  them  down.  Then  the  second  list  is  read  to  the  second 
instrument,  and  so  on. 

Such  words  are  difficult  to  distinguish,  especially  when  there 
is  no  context,  as  is  the  case  in  reading  an  arbitrary  list  of  words. 
The  receiving  party  should  be  required  to  write  down  the  words 
as  he  hears  them,  and  the  list  which  is  most  correct  according  to 
his  notes  will  probably  represent  the  work  of  the  best  instrument 
so  far  as  clearness  is  concerned.  It  is  only  by  a  consideration 
of  such  details  as  these,  simple  though  they  be,  that  an  unbiased 
opinion  can  be  formed  as  to  the  relative  merits  of  telephonic 
apparatus. 

Many  elaborate  experiments  have  been  performed  for  arriving 
at  the  comparative  merits  of  similar  instruments  depending  on 
the  quantitative  measurements  of  the  amplitude  of  vibrations,  the 
amount  of  current,  and  similar  quantities,  but  in  the  first  place 
the  apparatus  and  time  for  such  measurements  are  available  to 
but  few,  and  in  the  second,  place  it  is  doubtful  if  the  results 
obtained  are  as  reliable  as  those  obtained  by  carefully  following 
the  above  suggestions  in  a  conscientious  manner. 

Such  quantitative  experiments  are,  however,  of  great  impor- 
tance in  adding  to  our  knowledge  of  the  true  workings  of  the 
telephone,  thus  greatly  aiding  in  the  development  of  the  art  in 
general. 

A  series  of  experiments,  cited  by  Preece  and  Stubbs  and  per- 
formed by  the  administration  of  the  Swiss  telephone  department, 
is  of  great  interest.  In  this  test  a  good  Blake  transmitter  was 
used  throughout,  the  object  being  to  determine  the  best  of  a  set 
of  ten  induction  coils.  Table  I  gives  complete  data  concerning 
the  primary  and  secondary  windings  of  each  coil. 

The  results  obtained  over  five  different  lengths  of  line  are  shown 
in  the  right-hand  portion  of  the  table.  In  each  case  the  inten- 


AMERICAN   TELEPHONE  PRACTICE. 
TABLE    I. 


PRIMARY  WINDING. 

SECONDARY 
WINDING. 

RESULTS  FOR 
VARIOUS  LENGTHS  OF  LINE. 

of  Coil 

Cfl 

c 

O 

CO 

<*j 

<A 

t/5 

c 

o 

C/2 

1 

.31  mile 

38  miles 

49  miles 

53  miles  67  n 

liles 

VH 

ti 

P3 

O 

3 

M 

O 

1 

E 

°> 

0  J 

o 

°£ 

C  0) 

o 

4 

i 

ui 

V, 

6^ 

r 

"3 

su 

5 

'55 

c 

0> 

rt 

C 

i- 

rt 

£      S     £ 

S  :  « 

£ 

£ 

v 

c 

— 

C 

r? 

c     o 

c 

P  !  "^ 

u 

i 

61 

24 

•25 

1956 

35 

IOO 

•3 

•9 

9 

.0 

•3         7 

7 

.81     .: 

!          .9 

2 

62 

24 

•25 

3191 

35 

1  80 

•7 

•9 

o 

.1 

.0        o 

i  t. 

•3|      •' 

7          .O 

3 

62 

24 

•25 

4080 

35 

250       ||      .9 

•  q 

0 

•3 

.9        o 

.3!      •( 

3          .0 

4         116 

24 

•5° 

3952 

35 

250 

1.5 

7 

•5 

•3;           5 

*l      -S 

5 

230 

24 

I.  00 

3865 

35 

250 

1-3 

i  .0 

3i      -2       .1         3 

•  5  '.      •  ( 

si      -3 

6 

232 

24 

i  .20 

4420 

35 

300 

i  .5 

.     .9 

6-9-7         3 

.  6 

5!      -5 

7   I      295 

24 

1.50 

4278 

35 

300 

i  .3 

•9 

5 

•9 

.1        .  1 

•4        • 

3       -3 

8        368 

24 

2.OO 

4735 

35 

350 

1-3 

i  .0 

•5 

•  q 

.1           .0 

4[      . 

r     .2 

9 

368 

21 

I.I7 

4735 

29 

130.2   1 

i  .0 

.6 

•9 

•7       -4 

.6,   1.7       -3. 

10 

135° 

24 

10.00 

3950 

35 

400 

•3 

•3 

•3 

•5 

•3       -3 

•41     • 

1 

sity  and  clearness  of  the  Blake  transmitter  with  a  standard  coil 
was  taken  as  unity,  and  the  results  are  expressed  in  terms  of  this 
standard.  The  resistance  of  the  primary  wire  of  this  coil  was 
1.05  ohm  and  that  of  the  secondary  180  ohms.  It  will  be  no- 
ticed from  the  results  that  coils  Nos.  4,  6,  and  9  were,  all  things 
considered,  the  best,  while  coils  Nos.  i  and  10  were  very  inferior. 
The  table  also  shows  in  general  that  a  coil  that  was  good  for  a 
short  distance  was  also  good  for  a  long  distance,  and  this  is  per- 
haps the  most  instructive  lesson  to  be  gained  from  these  tests. 
It  is  hard  to  draw  any  definite  conclusions  from  the  performances 
of  the  various  coils  as  to  their  relative  merits  and  to  point  out 
why  coils  Nos.  4,  6,  and  9  should  give  better  results  than  the  others 
or  why  coils  Nos.  I  and  10  should  be  so  much  inferior.  It  shows, 
moreover,  that  good  results  maybe  obtained  with  the  same  trans- 
mitter and  with  coils  differing  widely  as  to  their  characteristics; 
this  being  shown  particularly  in  the  case  of  coils  Nos.  4  and  9, 
the  former  having  a  secondary  of  250  ohms  and  a  primary  of 
|  ohm,  while  the  latter  had  a  secondary  of  130  and  a  primary 
of  1.17  ohm.  The  coil  adopted  for  the  Blake  transmitter  in  this 
country  has  a  primary  winding  of  ^  ohm  and  a  secondary  of  250 
ohms,  which,  it  will  be  seen,  corresponds  exactly  to  coil  No.  4  in 
this  table,  which  gave  the  best  results.  The  tendency,  however, 
among  the  manufacturing  concerns  whose  practice  may  be  con- 
sidered the  best,  is  to  reduce  the  ratio  of  transformation  by 
making  the  secondary  windings  very  much  lower  than  was  form- 


INDUCTION  COILS.  57 

erly  the  case.  As  an  extreme  example  of  this,  it  may  be  cited 
that  the  coil  used  to  a  large  extent  with  the  solid-back  trans- 
mitter on  the  long-distance  lines  of  the  American  Telephone  and 
Telegraph  Co.  has  a  primary  of  .3  ohm  and  a  secondary  of 
but  14  ohms  resistance.  This  coil  is  provided  with  a  very  large 
core  composed  of  a  bundle  of  soft-iron  wires,  and  its  total  length 
between  the  cheeks  is  six  inches.1  The  results  obtained  leave  no 
doubt  as  to  the  efficacy  of  this  construction. 

The  particular  coil  to  be  used  with  any  style  of  transmitter 
and  battery  should  be  carefully  determined  experimentally  at 
the  start,  and  having  once  been  decided  upon,  should  not  be 
changed  except  for  very  positive  evidence  that  the  change  is  for 
the  better. 

The  determination  of  the  proper  size  and  dimensions  of  a 
standard  coil  is  no  easy  matter,  and  probably  the  best  way  is  by 
a  process  of  elimination.  When  carried  out  properly,  however, 
even  this  is  a  somewhat  expensive  and  tedious  operation.  Hav- 
ing decided  on  the  general  dimensions  of  the  core,  about  a 
dozen  of  them  should  be  made  up  and  wound  with  primary  coils, 
using  conductors  ranging  from,  say,  No.  18  to  No.  30  B.  &  S. 
gauge,  using  in  each  case  two  or  three  layers  of  these  wires  only. 
This  will  give  a  set  of  cores  all  alike,  having  primary  coils  rang- 
ing from  perhaps  \  of  an  ohm  to  8  ohms.  After  this  a  number 
of  secondaries  should  be  wound  on  spools,  adapted  to  slip  over 
the  primaries.  These  may  be  wound  to  resistances  ranging  from 
10  to  500  ohms,  always  using  a  large  enough  wire  to  approxi- 
mately fill  the  available  wire  space.  This  will  make  available  a 
larger  variety  of  induction  coils  than  would  probably  be  obtained 
in  any  other  way,  for  it  is  evident  that  each  primary  may  be 
used  with  each  one  of  the  secondaries. 

In  conducting  the  experiments,  one  of  the  primary  coils 
should  be  chosen,  and  the  results  tested  by  using  each  one  of 
the  secondaries  successively  in  connection  with  that  primary. 
The  best  of  these  combinations  should  be  noted,  and  then 
another  primary  should  be  tried  in  a  similar  manner  with  all  of 
the  secondaries.  In  like  manner  all  of  the  primaries  should  be 
tried  with  all  of  the  secondaries,  note  being  made  of  the  best 
combination  in  each  case.  In  this  the  best  secondary  for  each  of 
the  primary  coils  chosen  will  be  known,  and,  in  order  to  arrive  at 
the  final  result,  a  comparative  test  should  then  be  made  in  a 
similar  manner  with  each  of  these  combinations.  This  process 
may  be  carried  out  with  as  great  a  degree  of  refinement  as  time 
and  patience  will  permit ;  and  after  the  best  combination  has 


5«  AMERICAN    TELEPHONE   PRACTICE, 

been  found  for  any  particular  size  of  core,  the  entire  operation 
may  be  repeated  as  many  times  as  is  desired,  using  different  sizes 
of  core. 

Fig.  53  shows  a  sectional  view,  and  Fig.  54  a  perspective 
view,  of  a  coil  the  dimensions  of  which  were  determined  by  a 
method  not  unlike  that  just  described.  This  is  the  coil  used 
with  the  transmitter  of  the  Western  Telephone  Construction 
Co.,  shown  in  Fig.  43,  and  has  proven  itself  to  be  adapted  for 
almost  any  variety  of  work.  The  core,  C,  is  formed  of  a  bundle 
of  about  500  strands  of  No.  24  B.  &  S.  gauge  Swedish  iron 
wire,  and  is  4  inches  in  length  and  -f$  of  an  inch  in  diameter. 
The  spool  is  formed  of  a  thin  fiber  tube,  7^,  over  the  ends  of 


Figs.  53  and  54. — Section  and  Perspective  View  of  Induction  Coil. 

which  are  slipped  the  heads,  E,  of  similar  material,  the  parts 
being  glued  together.  On  this  core  are  wound  about  200  turns  of 
No.  20  single  silk-covered  wire.  This  is  two  layers  deep,  so  that 
the  ends  of  the  primary  both  emerge  from  the  same  end  of  the 
coil.  Over  the  primary  winding  are  wrapped  several  layers  of 
oiled  paper,  after  which  the  secondary  is  wound,  this  consisting 
of  about  1400  double  turns  of  No.  34  wire,  two  in  parallel. 
These  two  wires  are  wound  side  by  side  throughout  their  length, 
and  give  the  equivalent  area  of  one  No.  31  wire.  The  resistance 
of  the  primary  coil  is  .38  ohm  and  that  of  the  secondary  75 
ohms.  The  terminals  of  the  secondary  coil  are  shown  at  a  b 
and  a  b.  in  Fig.  53.  After  the  coil  is  wound,  the  small  wires 


INDUCTION   COILS. 


59 


of  the  secondary  are  attached  to  larger  wires  inside  of  the  spool- 
head,  so  that  the  danger  of  breakage  will  be  diminished.  These 
leading-out  wires  should  be  coiled  in  a  tight  spiral,  in  order  to 
avoid  breakage  and  also  to  give  a  considerable  length  of  wire  in 
making  connections  where  it  is  needed. 

A  coil  constructed  on  somewhat,  radical  principles  is  shown  in 
Fig.    55,  this  being  manufactured   by   the  Varley  Duplex   Mag- 


Fig-  55- — Varley  Induction  Coil. 

net  Co.  The  core  consists  of  a  bundle  of  small  soft-iron  cables, 
each'  cable  being  composed  of  seven  strands  of  rather  fine 
Swedish  iron  wire.  On  this  the  primary,  consisting  of  three 
layers  of  cotton-covered  magnet  wire,  is  wound.  The  secondary 
is  wound  in  two  sections,  as  shown,  and  the  right-hand  head  of 
the  spool  is  made  removable,  so  that  each  section  may  be  slid  on 
or  off,  as  needed,  in  making  repairs.  The  most  unique  feature  in 
this  coil  is  the  fact  that  bare  wire  is  used  in  winding  the  sec- 
ondary. These  coils  are  wound  by  special  machinery,  and  the 


Fig.  56.  —Manner  of  Winding  Varley  Coil. 

adjacent  convolutions  of  the  wire  are  held  apart  by  a  fine  thread 
of  silk  wound  alongside  and  parallel  with  the  wire,  as  shown  in 
Fig.  56.  A  layer  of  paper  is  introduced  between  each  layer 
of  wire,  and  in  this  way  the  insulation  is  made  complete.  The 
machines  for  winding  in  this  manner  have  been  perfected  with 
such  nicety  that  the  layer  of  paper  is  automatically  introduced 


60  AMERICAN   TELEPHONE   PRACTICE. 

between  each  winding  without  stopping  the  machinery,  which  is 
run  at  a  very  high  speed.  Considerably  more  wire  can  be  placed 
on  a  coil  in  a  given  space  than  with  the  ordinary  method  of 
winding ;  and,  of  course,  the  fact  that  bare  wire  is  used,  renders 
the  coil  cheaper. 

This  same  company  has  recently  carried  the  idea  of  sectional 


Fig.  57. — Transmitter  Mounted  on  Ann. 

windings  throughout  the  entire  field  of  telephone  work.  They 
construct  their  spools  in  such  manner  that  the  heads  may  be 
readily  removed  and  a  coil  replaced  without  the  necessity  of  re- 
winding. A  comparative  test  made  by  the  writer,  using  an 
induction  coil  wound  in  the  ordinary  manner  with  silk-covered 
wire  and  another  coil  wound  with  bare  wire  on  the  same  size  of 
core  and  with  the  same  resistance  of  primary  and  secondary, 


Fig.  58. — Induction  Coil  in  Base  of  Arm. 

showed  a  very  slight  advantage  in  favor  of  the  latter,  although 
the  experiment  was  not  carried  far  enough  to  warrant  the  con- 
clusion that  this  would  be  true  in  every  case. 

It  is  now  quite  common  to  mount  the  induction  coil  in  the 
base  of  the  arm  on  which  the  transmitter  itself  is  mounted,  such 


INDUCTION  COILS.  61 

construction  being  shown  in  Figs.  57  and  58.  This  base  and 
arm  are  made  of  cast  iron  joined  as  shown  in  such  manner  as  to 
allow  a  considerable  vertical  movement  of  the  transmitter,  in 
order  to  accommodate  it  to  the  heights  of  different  users.  The 
coil  is  sometimes  mounted  upon  the  back- board  of  the  telephone, 
but  a  more  desirable  method  is  to  mount  it  in  the  arm-base,  as 
shown,  the  various  terminals  being  brought  out  to  binding  posts 
on  the  front  of  the  base.  This  ^construction,  however,  is  bad, 
unless  well  carried  out,  and  great  pains  should  be  taken  in  insu- 
lating the  various  posts  and  wires  from  the  conducting  base.  A 
considerable  advantage  has  been  claimed,  due  to  the  presence  of 
the  iron  case  about  the  coil,  thus  rendering  the  magnetic  circuit 
more  complete.  This,  however,  is  a  point  of  doubtful  validity,  as 
it  may  be  claimed  with  equal  force  that  the  presence  of  the  case 
gives  rise  to  eddy  currents  which  would  have  a  detrimental 
effect.  As  a  matter  of  fact,  the  presence  of  the  case  has  little 
appreciable  effect  one  way  or  another  on  the  quality  of  the 
transmission. 


CHAPTER   VI. 

BATTERIES. 

IF  a  sheet  of  zinc  and  one  of  carbon  be  separated  from  each 
other  and  immersed  in  a  liquid  capable  of  chemically  attacking 
the  zinc,  a  difference  of  potential  will  at  once  be  formed  between 
the  two  plates.  If  the  two  plates  are  then  connected  together 
by  a  wire,  a  current  of  electricity  will  flow  from  one  to  the  other 
through  the  wire,  and  while  the  current  is  so  flowing  the  zinc  will 
be  eaten  away  by  the  solution  with  more  or  less  rapidity.  Such 
a  combination  is  called  a  voltaic  cell,  and  two  or  more  of  such 
cells  may  form  an  electric  battery.  Of  course  other  substances 
than  zinc  and  carbon  may  be  used,  it  only  being  necessary  that 
both  plates  be  of  conducting  material  and  that  one  of  them 
shall  be  of  such  a  nature  as  to  be  chemically  attacked  by  the 
fluid.  The  two  plates  of  the  cell  are  called  electrodes,  and  the 
solution  in  which  they  are  immersed  the  electrolyte. 

The  current  is  assumed  to  flow  from  the  plate  which  is  at- 
tacked through  the  electrolyte  to  the  one  which  is  not,  and 
therefore  in  the  cell  under  consideration  from  the  zinc  to  the 
carbon  plate.  The  plate  which  is  attacked  is  therefore  always 
called  the  positive  plate  or  electrode,  and  the  one  which  is  not 
attacked  the  negative. 

Starting  from  the  surface  of  the  zinc,  where  the  chemical 
action  is  taking  place,  the  current  flows  through  the  electrolyte 
to  the  surface  of  the  carbon  electrode,  thence  by  means  of  the 
wire  back  to  the  zinc  electrode. 

It  will  be  noticed  that  the  current  flows  from  the  carbon  to 
the  zinc  in  the  wire,  outside  the  electrolyte  ;  and  therefore  in 
order  to  make  the  terms  positive  and  negative  correspond  to 
ordinary  usage,  the  carbon  terminal  is  called  the  positive  pole 
and  the  zinc  terminal  the  negative  pole.  It  seems  at  first  a  little 
confusing  to  have  a  positive  pole  on  a  negative  plate,  and  a 
negative  pole  on  a  positive  plate  ;  but  if  the  direction  of  the 
current  be  kept  in  mind  as  being  always  from  positive  to 
negative,  no  confusion  will  arise. 

The  part  of  the  circuit  outside  of  the  battery  connecting  the 
two  poles  is  called  the  external  circuit.  The  internal  circuit  is 

62 


BA  TTERIES.  63 

of  course  through  the  two  electrodes  and  the  electrolyte,  and 
the  resistance  of  this  latter  path  is  called  the  internal  resistance 
of  the  battery. 

Zinc  forms  the  active  or  positive  element  of  the  great  majority 
of  primary  batteries,  while  the  negative  electrode  is  usually  of 
carbon  or  of  copper.  No  matter,  however,  of  what  materials 
the  electrodes  are  formed,  that  which  is  attacked  by  the  elec- 
trolyte while  the  battery  is  in  action  forms  the  positive  plate  of 
the  cell,  the  current  flowing  always  from  it  in  the  electrolyte. 

In  nearly  all  cases  hydrogen  is  liberated  from  the  electrolyte 
at  the  negative  plate — that  is,  at  the  plate  which  is  not  attacked. 
This  forms  a  film  over  the  surfaces  of  the  negative  electrode 
which,  unless  removed  or  destroyed,  tends  to  greatly  weaken  the 
strength  of  the  battery,  for  two  reasons :  First,  the  film  of  gas  is 
of  very  high  resistance  and  therefore  raises  the  internal  resist- 
ance of  the  battery  enormously,  thus  causing  a  correspondingly 
small  flow  of  current  ;  and  second,  the  gas  is  itself  attacked  by 
the  electrolyte,  hydrogen  having  almost  as  great  an  affinity  for 
the  oxygen  in  the  latter,  as  has  the  electrolyte  itself  for  the  zinc. 
This  causes  a  counter-electromotive  force  to  be  set  up  which  to 
a  large  extent  neutralizes  that  set  up  by  the  action  of  the  elec- 
trolyte with  the  zinc.  The  phenomenon  of  the  collection  of 
hydrogen  on  the  negative  electrode  in  a  cell  is  called  polariza- 
tion ;  and  it  is  necessary  to  adopt  some  means  to  prevent  it  to 
as  great  an  extent  as  possible,  as  otherwise  a  cell  would  become 
useless  after  a  very  short  period  of  use. 

The  LeClanche  type  of  battery,  which  has  been  and  still  is  used 
to  the  greatest  extent  for  telephone  work,  consists  of  a  carbon 
negative  electrode,  a  zinc  positive  electrode,  and  an  electrolyte 
of  a  solution  of  sal  ammoniac.  The  sal  ammoniac  attacks  the 
zinc,  forming  zinc  chloride  and  liberating  hydrogen  and  also 
ammonia  gas  on  the  surface  of  the  carbon.  In  order  to  get  rid 
of  the  polarizing  effects  due  to  the  hydrogen,  black  oxide  of 
manganese,  usually  in  small  lumps,  is  in  some  way  closely  asso- 
ciated with  the  carbon.  This  oxide  of  manganese  is  exceedingly 
rich  in  oxygen,  which  slowly  unites  with  the  free  hydrogen  to 
form  water.  In  use,  cells  of  this  type  polarize  rather  quickly, 
but  as  soon  as  the  external  circuit  is  opened  the  cell  slowly  re- 
covers, owing  to  a  combination  of  the  hydrogen  with  the  oxygen 
as  described  above.  This  cell  is  therefore  suitable  only  for  cases 
where  the  circuit  will  be  closed  for  a  few  minutes  at  a  time  ;  and 
as  this  is  exactly  the  condition  which  is  met  in  telephony,  it  has 
been  found  particularly  suitable  in  this  line  of  work. 


64 


AMERICAN    TELEPHONE  PRACTICE. 


The  cell  used  almost  exclusively  by  the  Bell  companies  is 
shown  in  Fig.  59- 

The  zinc  electrode  is  in  the  form  of  a  rod,  while  the  carbon 
electrode  is  imbedded  in  a  porous  pot  which  is  immersed  with  the 
zinc  in  the  electrolyte.  Around  the  carbon  within  the  porous 
pot  is  packed  a  mixture  of  black  oxide  of  manganese  and  broken 
carbon,  the  former  to  act  as  the  depolarizer  and  the  latter  to  give 
greater  conductivity  to  the  mixture  and  to  give  a  greater  surface 
to  the  carbon  electrode.  One  of  these  cells  is  almost  invariably 
found  in  connection  with  the  Blake  transmitter. 

A  better  form  of  cell  than  this  is   one  using  practically  the 
same    materials    for   its   various  parts,  designed    by  Mr.  M.  M. 
Hayden  of  the  Globe  Carbon  Works,  Ravenna,  O.     This  cell  is 
shown  in  Figs.  60  and  61,  the  latter  being  a 
sectional  view.     The    carbon    electrode   is  in 
'the  form  of  a  corrugated   hollow  cylinder,  I 
(Fig.  61),    which    engages  by   means    of    an 
internal  screw-thread  a  corresponding  thread 
on   the    under    side    of    a     carbon    cover,    2. 
Within  this  cylinder  is  a  mixture,  10,  of  broken 
carbon  and  black    oxide   of    manganese,   the 
latter  serving  as  a  depolarizer. 

The  zinc  electrode,  6,  is  in  the  form  of  a 
hollow  cylinder  almost  surrounding  the  car- 
bon electrode,  and  separated  therefrom  by 
means  of  heavy  rubber  bands  stretched  around 
the  carbon.  The  rod  forming  the  terminal 
of  the  zinc  passes  through  a  porcelain  bushing 
on  the  cover  plate,  so  that  a  short-circuit  can- 
not take  place.  The  terminal  pin,  8,  is  imbedded  in  a  hole,  4, 
in  the  carbon  cover,  by  first  heating  the  cover  to  a  high  degree 
and  then  pouring  in  melted  lead,  as  shown.  This  forms,  with 
the  nut,  7,  and  the  washer,  6,  a  very  secure  form  of  con- 
nector for  the  positive  pole.  Unless  some  such  precaution  as 
this  is  taken,  corrosion  soon  sets  in  around  the  metallic  con- 
nection to  the  carbon,  thus  causing  a  poor  connection.  The 
Hayden  cells  are  used  to  a  very  large  and  increasing  extent  by 
the  independent  telephone  companies.  They  have  an  electro- 
motive force  of  about  1.55  volts,  and  recuperate  very  quickly 
after  severe  use. 

Many  other  forms  of  sal-ammoniac  cells  are  in  common  use. 
Some  of  these  consist  merely  of  a  zinc  rod  hanging  in  the  center 
of  a  carbon  cylinder,  no  depolarizer  being  furnished.  In  other 


Fig.  59. — LeClanche 
Cell. 


BA  TTERIES.  65 

forms  the  carbons  have  molded  with  them  the  manganese  de- 
polarizer and  are  in  various  forms,  but  all  act  in  the  same 
general  way. 

The  advantages  of  the  LeClanche  type  of  cell  for  telephone 
work  are  many.  They  are  inexpensive  in  first  cost  and  in 
renewals.  They  are  very  cleanly,  giving  out  no  noxious  fumes 
and  containing  no  highly-  corrosive  chemicals.  They  require 
almost  no  attention,  the  addition  of  a  little  water  now  and  then 
to  replace  the  loss  due  to  evaporation  being  about  all  that  is 
generally  required.  They  give  a  rather  high  electromotive 
force  and  have  a  moderately  low  internal  resistance,  so  that 
they  are  capable  of  giving  a  considerable  amount  of  current  for 


Fig.  60. — Hayden  Cell. 

a    short    time,    and    lastly,    if    properly    made    they    recuperate 
quickly  after  polarization  due  to  heavy  use. 

To  set  up  and  maintain  cells  of  the  LeClanche  type  place  not 
more  than  four  ounces  of  prime  white  sal  ammoniac  in  the  jar. 
Fill  the  jar  one-third  full  of  water  and  stir  until  the  sal  ammoniac 
is  all  dissolved.  Then  place  the  carbon  and  zinc  elements  in 
place.  A  little  water  poured  in  the  vent-hole  of  the  porous-pot 
forms  will  tend  to  hasten  the  action.  Unless  a  cell  is  subject  to 
very  severe  use,  it  will  require  but  little  attention  if  it  is  a  good 
one.  Water  should  be  added  to  supply  loss  by  evaporation.  If 
the  cell  fails  to  work,  examine  its  terminals  for  poor  connections. 
If  the  zinc  is  badly  eaten,  replace  it  with  a  new  one.  If  this  fails 
to  improve  it,  throw  out  the  solution  and  refill  as  at  first.  If 


66 


AMERICAN    TELEPHONE  PRACTICE. 


now  the  cell  does  not  work  properly,  the  porous  pot  or  carbon 
element  may  be  soaked  in  warm  water,  and  if  this  gives  no 
better  results  they  should  be  replaced.  In  the  Hayden  cell,  the 
depolarizer  may  be  removed  by  unscrewing  the  carbon  from  the 
cover. 

The  Bell  Company  is  now  using  in  its  long-distance  work,  in 
connection  with  the  solid-back  transmitter,  another  form  of  cell 
known  as  the  "  Standard  "  Fuller.  In  this  the  positive  electrode 


Fig.  61. — Sectional  View  Hayden  Cell. 

is  a  heavy  block  of  zinc  molded  into  conical  form  around  a  heavy 
copper  wire,  which  forms  the  negative  pole.  The  negative  elec- 
trode is  a  block  of  carbon  hanging  through  a  slot  in  a  wooden 
cover.  The  separate  parts  are  shown  in  Fig.  62.  The  zinc  rests 
in  the  bottom  of  a  porous  cup  when  in  place.  The  electrolyte 
for  this  cell  is  made  as  follows  : 


Sodium  bichromate, 
Sulphuric  acid,     . 
Soft  water, 


6  ounces 
17  ounces 
56  ounces 


BA  TTERIES.  67 

Dissolve  first  the  sodium  bichromate  in  the  water  and  then  add 
slowly  the  sulphuric  acid.  (Never  pour  the  water  into  the  acid.) 
The  mixture  should  be  made  in  an  earthen  vessel,  or  if  in  a 
glass  jar  the  jar  should  be  placed  in  cold  water  in  order  to  pre- 
vent overheating. 

Another  solution  called  electropoin  fluid  may  be  used  as  the 
electrolyte  in  this  cell.  It  is  made  with  bichromate  of  potash 
instead  of  bichromate  of  sodium.^ 

The  cell  is  set  up  according  to  the  following  directions  : 

Place  the  quantity  of  solution  made  by  the  above  formula  in 
the  glass  jar. 

Put  one  teaspoonful  of  mercury  in  the  bottom  of  the  porous 


kjfc_— •*   ;  . 

Fig.  62. — Parts  of  "  Standard  "  Fuller  Cell. 

cup,  add  two  teaspoonfuls  of  common  salt,  place  the  zinc  in  the 
bottom  of  the  cup,  and  fill  to  within  two  inches  of  the  top  with 
soft  water. 

Place  the  porous  cup  in  the  jar  and  put  on  the  cover,  passing 
the  wire  from  the  zinc  through  the  hole  provided  for  it.  The 
cell  is  then  ready  for  use. 

The  active  element  in  the  electrolyte  in  this  cell  is  the  sul- 
phuric acid,  which  of  course  attacks  the  zinc.  The  bichromate  of 
sodium  or  of  potash  serves  as  a  depolarizer,  the  oxygen  in  it  com- 
bining with  the  hydrogen,  liberated  at  the  positive  pole,  to  form 
water. 


68 


AMERICAN    TELEPHONE   PRACTICE. 


The  specifications  for  this  cell,  as  used  by  the  New  York 
Telephone  Company  and  some  other  large  Bell  concerns,  are  in 
substance  as  follows  : 

One  cell  of  Standard  Battery  shall  consist  of  the  following  parts: 
I  glass  jar ;  I  wooden  cover ;  I  carbon  plate  with  binding  post 


,1 L 


1 


"LINE  OF  PARAFFINE 


:  I 

f 


..L 


Fig.  63.— Carbon  Plate  for  "  Standard  "  Fuller  Cell. 

and  locknuts ;  I  cast  zinc  ;  I  porous  pot — all  as  hereinafter 
specified. 

Glass  Jar:  The  glass  jar  shall  be  of  first  quality  flint  glass, 
cylindrical  in  form,  6  inches  in  diameter  and  8  inches  in  depth. 

Wooden  Cover:  The  cover  shall  be  of  clear  kiln-dried  white- 
wood.  It  shall  be  thoroughly  coated  with  two  coats  of  asphalt 
pamt,  and  be  of  such  dimensions  as  to  form  a  proper  cover  for 
the  jar. 

Carbon    Plate  :  The  carbon  plate  shall  be  of  the    form   and 


BA  TTERIES. 


69 


dimensions  shown  in  the  drawing  (Fig.  63).  It  shall  be  of  good 
quality,  homogeneous  and  free  from  flaws,  cracks,  and  other  de- 
fects, and  completely  carbonized.  Each  carbon  shall  be  provided 
with  a  clamp  of  the  form  and  dimensions  shown  in  the  drawing 
(Fig.  64).  The  parts  of  the  clamp  shall  be  of  bronze,  and  shall  be 
nickel-plated.  Before  attaching  the  clamp  to  the  carbon,  the 
carbon  shall  be  heated  to  a  temperature  of  at  least  250  degrees 
Fahrenheit,  and  the  top  portion  of  it,  to  the  extent  indicated  in 
the  drawing,  shall  be  immersed  in  paraffin  at  a  temperature  of 
about  250  degrees  Fahrenheit,  the  immersion  to  continue  until 
the  immersed  portion  of  the  carbon  is  saturated.  After  the  clamp 
is  attached  to  the  carbon,  but  before  the  locknuts  atre  in  place, 


Fig.  64.  —Details  of  Clamp  for  "  Standard  "  Fuller  Cell. 

the  carbon  shall  be  immersed  in  melted  paraffin  at  a  temperature 
less  than  170  degrees  Fahrenheit.  The  carbon  plate  is  then  to 
be  completed  by  attaching  the  locknuts. 

Cast  Zinc  :  The  zinc  shall  be  of  the  form  and  dimensions  shown 
in  the  drawing  (Fig.  65).  It  is  to  be  made  of  Rich  Hill  spelter. 
Cast  into  the  zinc  shall  be  a  soft  copper  wire  .1018  of  an  inch  in 
diameter  (No.  10  B.  &  S.  gauge).  The  zinc  and  the  copper  wire 
shall  be  amalgamated  to  a  height  of  4  inches. 

Porous  Pot :  The  porous  pot  shall  be  cylindrical  in  form,  3 
inches  in  diameter  and  7  inches  deep. 

The  "  Standard  "  Fuller  cell  made  according  to  the  above  speci- 
fications gives  an  E.  M.  F.  of  2.1  volts,  and  is  exceedingly  well 
adapted  for  heavy  telephone  service.  A  still  more  powerful 


7°  AMERICAN   TELEPHONE  PRACTICE. 

cell,  and   one  somewhat  more  convenient  to  handle,  is  shown 
in  (Fig.  66). 

In  this  the  zinc  is  very  heavy,  and  in  order  to  present  a  greater 
surface  to  the  electrolyte  has  a  horizontal  cross-section  in  the 
form  of  a  cross.  The  carbon  electrode  is  in  the  form  of  a  hollow 
cylinder  completely  inclosing  the  porous  pot.  The  carbon  cylin- 


NO.IOWIRE 


COMPLETE  ZINC 
TO  WEIGH  NOT  LESS 
THAN  18  OZ. 


Fig.  65.— Zinc  for  "  Standard  "  Fuller  Cell. 

der  has  a  flaring  top  provided  with  a  flange  which  fits  over  the 
upper  edge  of  the  glass  jar,  thus  forming  a  very  complete  cover 
for  the  entire  cell. 

The  following  are  the  data  given  by  the  Globe  Carbon  Co. 
concerning  the  main  points  of  this  form  of  Fuller  cell : 

E.  M.  F.,  2.1  volts. 

Current,  about  8  amperes. 


BATTERIES.  71 

Carbon,  4!  inches  diameter  by  8|  inches  over  all. 

Carbon  surface  exposed  to  solution,  156  square  inches. 

Zinc  weighs  2  pounds ;  2-J  inches  across  ;  total  length,  8  inches. 

Zinc  surface  exposed,  54  square  inches. 

Porous  cup,  3  inches  diameter,  7  inches  long. 

Jar,  6  inches  diameter,  8  inches  deep. 

Solutions  same  as  "  Standard  "  Fuller  cell. 

Cell,  complete,  weighs  8  pounds  12  ounces. 

The  internal  resistance  of  Fuller  cells  is  very  low,  especially  in 
the  cylindrical  carbon  type.  They  will  stand  for  several  months 
on  open  circuit  with  but  little  local  action. 

Formerly  three  cells  in  series,  giving  six  volts,  were  used  with 
the  solid-back  transmitter,  but  it  has  been  found  that  two  cells 
give,  all  things  considered,  as  good  or  better  results. 

Still  another  form  of  battery,  of  entirely  different  type,  is  shown 
in  Fig.  67.  This  is  known  as  the  gravity  battery,  and  is  used  to  a 


Fig.  66.— Parts  of  Globe  Fuller  Cell. 

very  large  extent  in  telegraph  service,  and  also  in  telephone 
work  where  it  is  necessary  to  have  a  small  but  constant  current 
always  flowing.  In  this  cell  the  negative  electrode  is  of  sheet 
copper,  3  strips  of  which  are  riveted  together  at  their  centers, 
after  which  the  ends  are  bent  outwardly,  so  as  to  present  a  large 
surface  to  the  electrolyte.  The  zinc  is  in  the  form  of  a  "  crow 
foot,"  cast  with  a  lug  adapted  to  hook  over  the  edge  of  a  glass 
jar.  In  setting  up  this  battery  the  copper  is  first  put  in  place  in 
the  bottom  of  the  jar.  Sulphate  of  copper,  or  blue  vitriol,  as  it  is 
called,  is  then  filled  in  around  the  copper  to  a  height  almost  suffi- 
cient to  cover  it.  The  jar  is  then  filled  with  water  and  the  zinc 
put  in  place. 

In  this  battery  sulphuric  acid  is  formed,  which  attacks  the  zinc 


72  AMERICAN   TELEPHONE  PRACTICE. 

to  produce  zinc  sulphate.  This  fluid  is  lighter  in  weight  than 
the  solution  of  copper  sulphate  and  therefore  occupies  the  upper 
portion  of  the  cell.  The  fact  that  the  two  solutions  in  this 
battery  are  kept  apart  by  gravity  instead  of  by  the  use  of  a 
porous  pot,  as  in  the  Fuller  cell,  is  accountable  for  the  name, 
"  gravity  cell."  As  the  zinc  sulphate  is  colorless,  while  the  copper 
sulphate  is  of  a  dark-blue  color,  the  separating  line  between  the 
two  liquids  is  easily  distinguished.  This  line  is  termed  the  "blue 
line,"  and  should  be  kept  about  midway  between  the  copper  and 
the  zinc.  If  the  blue  line  rises  too  high,  so  as  to  come  in  contact 
with  the  zinc,  it  should  be  lowered.  This  can  be  done  by  short- 
circuiting  the  battery  for  a  short  time,  or  by  drawing  off  some  of 
the  blue  fluid  with  a  siphon  and  filling  in  with  water  or  with  zinc 
sulphate  from  another  battery.  In  cases,  however,  where  the 
battery  is  in  constant  use,  it  very  rarely  happens  that  the  blue 


Fig.  67.— Gravity  Daniell  Cell. 

line  reaches  too  high  a  level,  and  the  reverse  is  more  likely  to 
take  place.  If  the  blue  line  reaches  the  upper  portion  of  the 
copper,  more  crystals  of  bluestone  should  be  dropped  in,  and  if 
this  does  not  remedy  the  difficulty  some  of  the  zinc  sulphate 
from  the  top  of  the  cell  should  be  siphoned  out  and  replaced  by 
clear  water.  These  batteries  are  very  satisfactory  for  closed- 
circuit  work,  but  are  not  well  adapted  for  telephone  work  in  gen- 
eral on  account  of  their  high  internal  resistance. 

When  a  battery  is  on  open  circuit  there  should  be  no  action 
between  the  electrolyte  and  the  zinc.     This  would  be  the  case 


BATTERIES.  73 

were  it  economical  to  use  perfectly  pure  zinc,  but  inasmuch  as 
commercial  zinc  always  contains  impurities,  frequently  consisting 
of  other  metals,  a  local  galvanic  action  is  set  up,  the  impurities 
forming  with  the  zinc  minute  galvanic  couples.  In  order  to  re- 
duce this  action  to  a  minimum,  it  is  advisable,  especially  in  such 
cells  as  the  Fuller,  to  amalgamate  the  zinc — that  is,  to  coat  it  with 
mercury.  This  seems  to  -form'a  perfectly  homogeneous  surface 
to  the  zinc,  which  prevents  local  action.  The  fact  that  this  local 
action  takes  place  on  account  of  impurities  in  the  zinc  makes 
it  very  clear  that  the  quality  of  metal  used  is  a  matter  of  very 
considerable  importance. 

STORAGE   BATTERIES. 

If  two  plates  of  lead  are  immersed  in  a  weak  solution  of  sul- 
phuric acid,  no  difference  of  potential  will  be  established  between 
them,  because  the  acid,  if  it  acts  on  them  at  all,  does  so  to  an 
equal  extent  on  each  plate.  If  now  an  electric  current,  as  from 
a  battery  or  a  direct-current  dynamo,  is  sent  through  the  two 
plates  and  the  solution  between  them,  a  redistribution  of  ma- 
terials will  take  place  in  the  cell.  The  electrolyte  will  be  decom- 
posed, the  oxygen  in  it  forming,  with  the  plate  to  which  the 
positive  terminal  of  the  charging  source  is  connected,  lead  per- 
oxide;  while  hydrogen  is  liberated  at  the  plate  to  which  the 
negative  terminal  is  connected.  On  disconnecting  the  source  of 
current,  the  cell,  which  was  before  incapable  of  producing  a 
difference  of  potential,  is  found  able  to  drive  a  current  through  a 
circuit  formed  by  connecting  its  poles  together  by  a  wire  or  any 
other  conductor.  The  combination  has  become  a  voltaic  couple. 
The  current  from  this  couple  always  flows  in  a  direction  opposite 
to  that  of  the  charging  current. 

The  cell,  consisting  of  two  lead  plates  in  a  solution  of  sulphuric 
acid,  was  devised  by  Gaston  Plante,  and  is  the  prototype  of  all 
modern  storage  batteries  or  accumulators.  Nearly  all  commer- 
cial cells,  of  which  there  are  many  good  ones,  have  the  plates 
coated  with  some  compound  of  lead,  rich  in  oxygen.  This  is 
changed  by  the  charging  current  into  lead  peroxide  on  the  posi- 
tive plate,  and  to  spongy  lead  on  the  negative. 

In  storage  cells  of  considerable  size  it  is  customary  to  use 
more  than  two  plates,  all  the  positive  plates  being  connected 
together  by  a  heavy  strip  of  lead,  and  likewise  all  the  negative 
plates  by  another  similar  strip.  There  is  usually  one  more  of  the 
negative  than  of  the  positive  plates,  the  arrangement  being  such 
that  the  plates  are  alternately  positive  and  negative. 


74  AMERICAN    TELEPHONE  PRACTICE. 

The  setting-up  and  operating  of  storage  batteries  is  a  very 
simple  matter,  yet  there  are  a  few  mistakes  to  be  guarded  against, 
which  if  made  are  liable  to  injure  or  ruin  the  battery.  The  elec- 
trolyte is  usually  formed  of  four  or  five  parts  of  water  to  one  of 
acid.  These  should  be  mixed  in  an  earthenware  vessel  by  slowly 
pouring  the  acid-into  the  water,  and  not  the  water  into  the  acid. 

In  charging  storage  batteries  the  positive  terminal  of  the 
dynamo  or  other  source  of  current  is  connected  to  the  positive 
pole  of  the  battery,  and  the  negative  terminal  of  the  dynamo  to 
the  negative  pole  of  the  battery.  A  simple  test,  and  the  most 
reliable  one,  for  determining  which  is  the  positive  pole  of  any 
source  of  current  is  to  dip  wires  leading  from  both  terminals  into 
a  small  vessel  containing  slightly  acidulated  water.  Bubbles  of 
gas  will  be  given  off  from  each  wire,  but  at  a  very  much  higher 
rate  from  the  wire  leading  to  the  negative  pole  than  from  that 
leading  to  the  positive.  The  poles  of  the  charging  dynamo 
should  always  be  determined  with  absolute  certainty  before  con- 
nection is  made  to  the  terminals  of  the  storage  battery,  for  a 
reversal  in  the  connections  is  very  likely  to  ruin  the  battery. 

The  manufacturers  of  storage  batteries  usually  furnish  direc- 
tions concerning  the  proper  rate  of  charge  and  discharge  for  a 
battery  of  a  given  size.  These  should  be  followed  as  closely  as 
conditions  will  allow. 

The  most  accurate  method  of  determining  the  condition  of  a 
cell  is  by  the  use  of  a  hydrometer  for  measuring  the  density  of  the 
electrolyte.  It  is  usual  to  have  the  normal  density  of  the  solution 
about  i .  1 80 ;  when  it  becomes  as  low  as  1 .  1 70  the  cell  may  be  con- 
sidered fully  discharged,  and  when  as  high  as  1.250  fully  charged. 
These  figures  will  vary  somewhat  with  different  forms  of  battery. 

Water  only  should  be  added  to  replace  loss  by  evaporation, 
while  spilled  solution  must  be  replaced  by  the  regular  acid  solu- 
tion according  to  formula. 

The  extremely  low  internal  resistance  of  storage  batteries,  and 
the  fact  that  their  voltage  is  high  (2  volts)  and  constant  and 
that  they  are  not  subject  to  polarization,  make  them,  all  things 
considered,  the  ideal  source  of  current  for  telephone  work.  They 
are  being  largely  used  for  supplying  the  operators'  transmitters 
in  large  central  offices.  They  are  much  more  economical  in 
operation  than  any  form  of  primary  cell,  inasmuch  as  there  is 
practically  no  consumption  whatever  of  the  materials  in  the  cell 
itself,  it  depending  of  course  for  its  energy  on  some  outside 
source.  Their  ease  of  manipulation  and  general  cleanliness  and 
reliability  are  also  strong  points  in  their  favor. 


CHAPTER  VII. 

CALLING   APPARATUS. 

So  far  we  have  dealt  solely  with  the  apparatus  by  which  the 
actual  transmission  of  speech  is  accomplished.  While  these  are,  of 
course,  the  most  vital  parts  of  a  complete  telephone,  they  would 
be  of  little  use  were  not  means  provided  whereby  one  party  might 
call  the  attention  of  another  in  order  to  bring  about  a  conversa- 
tion. Many  attempts  have  been  made  to  devise  telephone  in- 
struments capable  of  reproducing  speech  so  loudly  that  one  has 
only  to  call  into  the  transmitter  in  order  to  attract  the  attention 
of  a  party  at  the  other  end  of  the  wire.  Such  attempts  have  so 
far  resulted  practically  in  failure,  and  this  is  perhaps  fortunate, 
as  one  of  the  most  convenient  features  of  telephones  to-day  is 
that  a  conversation  can  be  carried  on  in  secrecy,  at  least  so  far 
as  the  receiving  is  concerned. 

Ordinary  vibrating  bells,  using  current  derived  from  a  battery, 
were  at  first  used  for  calling,  and  as  the  battery  for  operating  the 
transmitters  could  also  be  used  for  this  purpose,  this  plan  seemed 
to  offer  many  advantages.  It  was  found,  however,  that  the 
amount  of  energy  furnished  by  a  telephone  battery  was  insuffi- 
cient to  operate  call-bells  at  great  distances.  Of  course,  practi- 
cally as  high  voltage  as  was  desired  could  be  obtained,  by  using 
induction  coils  and  causing  induced  currents  from  the  secondary 
to  pass  out  over  the  line.  This,  however,  reduced  the  current  in 
the  same,  proportion  as  it  raised  the  voltage,  leaving  the  amount 
of  energy  the  same. 

What  is  known  as  a  "  magneto-generator  "  is  now  almost  uni- 
versally used  among  the  independent  companies,  and  until  re- 
cently by  the  Bell  Company.  It  is  the  simplest  known  form  of 
the  dynamo,  and  consists  of  an  armature  of  the  Siemens  type, 
wound  with  many  coils  of  fine  wire,  and  so  mounted  as  to  enable 
it  to  be  rapidly  revolved  between  the  poles  of  a  permanent  horse- 
shoe magnet.  Its  theory  of  action  is  very  simple  and  depends 
on  the  principles  of  magneto-electricity  discovered  by  Faraday 
and  Henry,  and  pointed  out  in  a  previous  chapter — that  if  the 
number  of  lines  of  force  passing  through  a  closed  coil  be  varied, 
currents  of  electricity  will  be  generated  in  this  coil,  the  direction 


76 


AMERICAN    TELEPHONE  PRACTICE. 


of  these  currents  depending  upon  the  direction  of  the  lines  of 
force  and  on  whether  their  number  is  decreasing  or  increasing. 

In  Fig.  68  is  shown  a  simple  loop  of  wire,  a,  which  may  be  re- 
volved about  a  horizontal  axis  in  the  field  of  force  of  the  per- 
manent magnet.  The  horizontal  arrows  represent  the  direction 
of  the  lines  of  force  set  up  by  the  magnet  through  the  loop. 
Suppose  the  loop  to  be  turned  in  the  direction  of  the  curved 
arrow.  When  it  is  in  the  horizontal  position  no  lines  of  force 
will  pass  through  it.  As  it  approaches  the  position  shown  by 
the  full  line  it  will  include  a  larger  and  larger  number  of  these 
lines.  The  current  induced  in  the  coil  will  then  be  in  the  direc- 
tion indicated  by  the  arrows,  x,  and  will  so  continue  until  the 
loop  is  in  its  vertical  position.  The  number  of  lines  passing 


X'    '      \ 


\    ,/  X 


a 


Fig.  68. — Field  of  Force  in  Magneto-Generator. 

through  the  loop  then  begins  to  decrease,  and  the  current  there- 
fore takes  the  opposite  direction,  as  indicated  by  the  arrows,  s. 
The  current  increases  in  strength  in  this  new  direction  until  the 
coil  is  horizontal.  At  this  point  the  rate  at  which  the  number  of 
lines  through  the  coil  is  changing  is  greatest,  and  the  current 
is  therefore  a  maximum.  As  the  coil  passes  through  the 
horizontal  position  the  number  of  lines  passing  through  it  begins 
to  increase  again.  This  would  cause  another  change  in  the 
direction  of  the  current,  were  it  not  for  the  fact  that  the  direc- 
tion of  the  lines  of  force  through  the  coil  also  changes.  The 
same  events  take  place  during  the  next  half-turn,  when  the  coil 
is  dn  the  position  from  which  it  started. 

We   thus  see  that  the  current  generated  is  an  alternating  o»e, 
changing  its  direction  twice  during  every  revolution. 


CALLING  APPARATUS. 


The  armature,  instead  of  having  a  single  turn  of  wire,  as  in 
Fig.  68,  has  a  great  number  of  turns  of  fine  wire  wound  on  a  cast- 
iron  core  of  the  form  shown  in  Fig.  69.  In  this  figure,  A  repre- 


Fig.  69. — Armature  of- Magneto-Generator. 

sents  a  shuttle-shaped  core  of  cast  iron,  on  which  the  coils  of 
wire,  w,  are  wrapped.  One  end  of  the  wire  forming  the  coils  is 
fastened  to  the  pin,  /,  which  is  fastened  to  and  is  in  metallic 
connection  with  the  core,  A.  The  other  end  is  fastened  to  the 
pin,  p ',  which  is  insulated  from  the  core,  but  connects  with  the 
pin,  Cj  projecting  from  the  end  of  the  armature  shaft  and  is  in- 
sulated therefrom  by  the  fiber  bushing,  b.  Projections,  a  a,  in- 
tegral with  the  core,  are  turned  down  to  form  bearings  for  the 
armature.  A  pinion,/,  is  carried  on  the  end  of  the  shaft,  in  order 


^Ss^ 


Fig.  70.— Diagram  of  Generator  and  Bell. 

to  transmit   to  the  armature  the  motion,  received  from  a  large 
driving-gear  wheel  with  which  it  meshes. 

A  magneto-generator  in  connection  with  a  call-bell  is  shown 
diagrammatically  in  Fig.  70.  To  the  poles  of  the  permanent 
magnets,  N  5,  of  the  generator  are  attached  cast-iron  pole- 
pieces,  P  P,  bored  out  so  as  to  allow  the  armature,  A,  to  turn 


78  AMERICAN    TELEPHONE  PRACTICE. 

freely  between  them.  The  bearings  of  the  armature  are  usually 
mounted  on  brass  plates  firmly  attached  to  the  ends  of  the  pole- 
pieces,  but  not  shown  in  this  figure.  By  means  of  a  crank 
attached  to  a  suitable  gear  wheel  engaging  a  pinion  on  the 
armature  shaft,  the  armature  may  be  made  to  turn  rapidly. 

As  the  currents  generated  are  alternating,  a  polarized  bell  or 
ringer  is  needed.  C  C  are  the  two  coils  of  an  electromagnet. 
Pivoted  in  front  of  the  poles  of  this  magnet  is  a  soft-iron  arma- 
ture, A',  carrying  a  hammer,  //,  on  the  end  of  a  thin  rod  extend- 
ing at  right  angles  from  its  center.  A  permanent  magnet,  N  S, 
is  so  mounted  as  to  magnetize  by  induction  the  armature,  A',  and 
the  cores  of  the  coils,  C  C. 

The  two  poles   of  the   electromagnet  will   thus  have  a  given 


Fig.  71. — Complete  Magneto-Generator. 

polarity,  say,  north,  while  the  two  ends  of  the  armature  will 
have  an  opposite  polarity,  south.  As  a  result,  the  armature  will 
have  a  tendency  to  stick  to  one  pole  or  the  other  of  the  magnets. 
The  two  coils  are  oppositely  wound,  and  when  a  current  passes 
through  them  it  strengthens  the  magnetism  of  one  pole  and 
weakens  that  of  the  other.  The  next  instant  the  current  reverses, 
and  the  strong  pole  becomes  the  weaker,  and  vice  versa.  As  a 
result  the  armature  vibrates  with  each  reverse  of  current  and 
causes  the  hammer,  H,  to  strike  the  bells,  B  B.  A  complete 
magneto-generator  and  call-bell,  mounted  in  a  box,  is  shown  in 
Fig.  71.  The  magnets  of  the  call-bell  are  mounted  on  the  inside 
of  the  lid,  the  hammer  extending  through  a  hole  therein  to 


CALLING  APPARATUS. 


79 


strike  the  gongs,  on  the  outside.  Fig.  72  shows  one  of  the 
commercial  forms,  and  a  very  efficient  one,  of  call-bell  mechanism. 
The  forms  of  ringers  used  by  different  manufacturers  differ 
widely  ;  but  all  depend  on  the  same  principles  for  their  mode  of 
action. 

The  armatures  of  ordinary  hand  generators  are  usually 
wound  to  resistances  varying  from  300  to  650  ohms.  The  resist- 
ance of  the  ringer  coils  is  usually  from  75  to  100  ohms,  but  is 
sometimes  as  high  as  5000  ohms,  varying  according  to  their  use. 

The  standard  generator  and  ringer  for  ordinary  exchange 
work  are  so  wound  that  the  generator  will  ring  its  own  bell,  or 
another  like  it,  through  a  resistance  of  10,000  ohms.  Such  an 
outfit  is  spoken  of  as  a  io,ooo-ohm  magneto,  and  the  10,000  re- 


Fig.  72. — Polarized  Ringer. 

fers  not  to  the  resistance  of  the  bell  magnets  or  the  generator  arma- 
ture, as  is  often  supposed,  but  to  the  external  resistance  through 
which  they  will  successfully  work. 

With  a  given  magnetizing  force,  as  for  instance  that  set  up  by 
a  permanent  magnet,  the  number  of  lines  of  force  extending 
from  one  pole  of  the  magnet  to  the  other  will  depend  on  the 
material  between  the  poles  and  also  on  the  distance  between 
them.  Certain  substances,  if  placed  in  a  magnetic  field  of  force, 
will  have  set  up  in  them  a  vastly  greater  number  of  lines  of 
force  than  would  air  when  subjected  to  the  same  magnetizing 
force.  Such  substances,  in  which  a  given  magnetizing  force  will 
produce  a  high  degree  of  magnetization,  are  said  to  possess  a 
high  degree  of  permeability.  The  permeability  of  a  substance  is 
expressed  numerically  by  the  ratio  of  the  number  of  lines  of  force 
set  up  in  a  given  area  of  it  by  a  given  magnetizing  force  to  the 
number  set  up  in  the  same  area  in  air  by  the  same  magnetizing 


8o  AMERICAN    TELEPHONE  PRACTICE. 

force.  Thus,  if  the  given  magnetizing  force  sets  up  50  lines  of 
force  per  square  inch  in  air  and  20,000  lines  per  square  inch  in  a 
piece  of  wrought  iron,  the  permeability  of  the  iron  would  be 

-  =  400.     The  permeability  of  air  is  always  taken  as  unity. 

Iron,  in  all  of  its  forms,  is  by  far  the  most  permeable  of  all 
metals,  and  even  among  the  various  grades  of  iron  there  is  a 
great  difference  in  this  respect.  Soft  wrought  iron  is  much  more 
permeable  than  cast  iron,  and  cast  iron  much  more  so  than  hard 
steel. 

The  great  point  in  the  design  of  magneto-generators,  as  in 
fact  in  dynamo  design  in  general,  is  to  cause  as  great  a  number 
of  lines  of  force  as  possible  to  pass  through  the  core  of  the 
armature.  In  the  design  of  ordinary  dynamos,  where  the  field 
is  composed  of  electromagnets,  the  magnetizing  force  can  be 
varied  almost  at  will  by  subjecting  the  field  to  the  influence 
of  a  great  number  of  ampere-turns. 

In  the  design  of  magneto-generators,  however,  the  strength  of 
the  field,  when  once  determined,  is,  for  all  practical  purposes,  in- 
variable, as  the  strength  of  the  magnets  is  in  no  wise  dependent 
on  the  current  generated.  Obviously,  therefore,  the  only  re- 
course, in  bettering  the  efficiency  of  the  machine  in  this  respect, 
is  to  use  as  fine  a  grade  of  iron  as  possible  in  the  armature,  and  to 
so  design  it  as  to  present  a  path  of  as  small  resistance  as  possible 
to  the  flow  of  the  magnetic  lines.  Not  enough  attention  has 
been  given  to  this  point,  and  often  a  poor  grade  of  cast  iron 
which  was  allowed  to  chill  after  casting  and  thus  become  ex- 
ceedingly hard,  has  been  used  in  constructing  generator  arma- 
tures. Fortunately,  however,  a  very  hard  grade  of  iron  is  very 
difficult  to  turn  in  a  lathe,  especially  in  this  particular  form,  and 
this  has,  indirectly,  made  some  manufacturers  seek  fairly  soft,  uni- 
form iron  for  this  purpose. 

A  cast  armature,  even  though  soft,  is  subject  to  another  ob- 
jection, in  that  eddy  currents  are  generated  in  the  core,  which, 
of  course,  interfere  greatly  with  the  efficiency  of  the  machine. 
In  order  to  do  away  with  both  of  these  objections,  some  com- 
panies are  now  building  laminated  armatures,  composed  of  soft 
sheet-iron  punchings  about  T1^-  of  an  inch  or  less  in  thickness, 
clamped  together  on  a  central  shaft  which  forms  the  spindle  of 
the  armature.  These  laminated  armature  cores  are,  when  com- 
pleted, of  about  the  same  shape  as  the  cast  core,  and  the  wire  is 
wound  on  them  in  the  ordinary  way. 

After   the    armature    core,    however    formed,    is   complete,    it 


CALLING  APPARATUS.  81 

should  be  thoroughly  insulated  by  paper  and  cotton  cloth,  held 
in  place  by  some  insulating  adhesive  such  as  shellac,  after  which 
it  is  placed  in  a  winding  machine  and  wound  with  the  required 
number  of  turns. 

The  winding  should  be  of  the  largest  size  of  wire  that  will  give 
the  desired  number  of  turns,  but  the  wire  space  should  not  be  so 
completely  filled  as  to  cause  the1  wire  to  bulge  out  and  strike  the 
pole-pieces  of  the  generator  in  its  rotation,  thus  wearing  away  the 
insulation  and  frequently  breaking  the  wire  itself. 

It  is  a  commonly  expressed  opinion  that  the  turns  of  wire 
near  the  center  of  the  armature  coil  are  of  little  or  no  value  in 
producing  electromotive  force.  This,  however,  is  not  the  case, 
for  the  permeability  of  iron  is  so  much  greater  than  that  of 
air  that  nearly  all  of  the  lines  of  force  due  to  the  permanent  mag- 
nets of  the  generator  will  pass  through  the  shank  of  the  core  in- 
stead of  leaking  around  through  the  air  space.  Of  course,  in 
order  to  pass  through  this  shank,  they  must  also  pass  through  the 
inside  turns  as  well  as  those  nearer  the  surface. 

The  question  of  permanent  magnets  is  a  puzzling  one,  prin- 
cipally because  very  little  seems  to  be  known  concerning  the 
kind  of  steel  best  adapted  for  this  purpose.  Makers  of  steel  can- 
not or  will  not  reproduce  the  quality  of  samples  of  steel  given 
them,  even  after  careful  chemical  analyses.  It  may  be  said, 
however,  that  a  few  makers  of  this  steel  are  able  to  turn  out  year 
after  year  large  quantities  of  very  uniform  steel  for  this  purpose, 
which  is  capable  of  giving  very  satisfactory  results.  If,  however, 
a  sample  of  one  maker's  steel  is  given  to  another  to  analyze  and 
reproduce,  the  result  is  usually  failure.  It  has  been  the  experi- 
ence of  the  writer  that  the  only  way  to  procure  a  good  magnet 
steel  is  to  test  all  of  the  samples  obtainable,  and,  having  found  a  sat- 
isfactory steel  which  the  manufacturer  is  able  to  produce  in  large 
quantities,  to  stick  to  that  particular  grade.  It  may  be  said 
further  that  the  more  expensive  grades  of  steel  are  not  by  any 
means  capable  of  producing  the  best  magnets;  and  frequently 
where  a  manufacturer  is  paying  ten  to  twelve  cents  per  pound 
for  magnet  steel,  a  little  experimenting  would  enable  him  to  find 
something  which  would  give  as  good  or  even  better  results  at 
from  two  and  one-half  to  five  cents  per  pound. 

The  usual  method  of  treating  steel  for  making  permanent  mag- 
nets is  to  cut  it  in  the  desired  lengths  and,  if  the  cross-section  be 
not  too  heavy  and  the  form  not  too  complicated,  to  bend  it  in  a 
special  former  while  cold.  It  is  then  heated  to  a  light  cherry- 
red  in  a  rather  slow  fire  and  then  grasped  by  a  special  pair  of  tongs 


82 


AMERICAN    TELEPHONE   PRACTICE. 


iii  such  manner  that  it  will  not  bend  from  the  desired  shape, 
and  plunged  into  a  tank  of  cold  running  water,  being  kept  in  vio- 
lent agitation  during  the  entire  time  of  cooling.  All  parts  of  the 
tongs  which  come  in  contact  with  the  magnet  during  this  process 
should  be  bored  as  full  of  holes  as  the  required  strength  will  per- 
mit in  order  that  the  water  my  have  free  access  to  all  portions  of 
the  steel.  After  the  bar  has  hardened  it  is  magnetized  by  strok- 
ing it  several  times  across  the  poles  of  a  very  powerful  electro- 
magnet. The  pole-pieces  of  this  magnet  should  be  sufficiently 
close  together  to  allow  one  leg  of  the  horseshoe  magnet  to  rest 
upon  the  north  pole  and  the  other  upon  the  south  pole.  In 
some  cases  the  bars  are  magnetized  by  inserting  them  in  a  sole- 
noid, but  probably  the  best  results  are  obtained  by  the  method 
of  stroking. 

Until  recently  cast  iron  was  the  only  material  used  for  pole- 
pieces  in  magneto-generators,  and  many  good  generators  are 
now  constructed  with  that  material.  A  number  of  generators, 


Fig.  73. ---Detail  of  Generator  Pole-Pieces. 

however,  have  recently  been  produced  using  soft  sheet-iron  pole- 
pieces  stamped  and  formed  into  the  desired  shape.  This  forms 
a  cheaper  pole-piece  than  can  be  procured  by  the  use  of  cast  iron, 
because  the  latter  must  necessarily  be  subjected  to  a  consider- 
able amount  of  machine  work,  such,  for  instance,  as  the  boring  of 
the  concave  cylindrical  surfaces  between  which  the  armature  re- 
volves. A  point  in  favor  of  the  cast-iron  pole-pieces  is  that  the 
air  gap  may  be  made  much  smaller  because  of  the  greater  accuracy 
of  this  bore  than  can  be  secured  by  the  use  of  punched  sheet-metal 
pole-pieces.  An  argument  in  favor  of  the  latter,  however,  is  that 
the  quality  of  iron  is  much  better,  and  this  is,  of  course,  of  some 
advantage,  but  not  so  great  as  it  would  at  first  appear,  because  the 
flow  of  magnetic  lines  through  these  pole-pieces  is  always  in  the 
same  direction  ;  and  the  loss,  therefore,  due  to  a  lack  of  permea- 
bility in  the  cast-iron  pole-pieces  is  probably  fully  offset  by  the 
greater  cross-section  of  iron  available  for  the  lines  to  traverse 
and  also  by  the  smaller  air  gap. 

In   the  construction  shown  in   Fig.   73   the  pole-pieces  are  cf 


CALLING  APPARATUS.  83 

cast  iron  firmly  secured  together  by  shouldered  brass  rods. 
After  being  thus  fastened  together  they  are  bored  out  with  a 
special  tool,  after  which  the  magnets  are  put  in  place  and 
clamped  by  any  suitable  means.  This  is  a  very  good,  although 
somewhat  expensive,  construction  when  properly  done. 

The  efficiency  depends  to  a  considerable  extent  upon  the  form 
of  the  current  wave  generated  [ by  the  machine.  This  is  gov- 
erned largely  by  the  relation  between  the  width  of  the  pole  of 
the  armature  and  the  distance  between  the  flat  surface  of  the 
generator  pole-pieces.  In  Fig.  73  the  best  relation  between 
these  dimensions  is  illustrated  quite  clearly.  It  will  be  noticed 
in  the  figure  at  the  left  that  the  curved  portion  of  the  armature 
pole  exactly  corresponds  to  the  concave  portion  of  the -pole- 
pieces,  while  in  the  figure  at  the  right,  which  shows  the  armature 
in  a  different  position,  the  poles  of  the  armature  are  just  suffi- 
cient in  width  to  bridge  across  the  space  between  the  pole-pieces 
without  overlapping. 

The  sine  wave  has  been  found  to  be  most  efficient  in  the 
ringing  of  magneto-bells,  especially  on  lines  of  considerable 
length  and  possessing  a  high  degree  of  self-induction  and  capacity  ; 
and  the  relation  between  the  armature  poles  and  the  pole-pieces 
shown  in  the  above  figure  gives  the  nearest  approximation  to 
this  form  of  wave.  Where  the  armature  poles  do  not  fill  the 
space  between  the  pole-pieces,  the  current-  curve  will  have  four 
distinct  humps  in  each  complete  cycle.  There  will  be  a  break  in 
the  magnetic  circuit  just  as  the  armature  pole  leaves  the  pole- 
piece  on  one  side,  which  will  cause  a  sharp  fluctuation  in  the 
electromotive  force  ;  and  another  sharp  fluctation  will  occur  im- 
mediately after,  when  the  opposite  points  of  the  armature  poles 
approach  the  corners  of  the  pole-pieces.  These  two  fluctuations 
will  occur  twice  in  each  cycle.  When  the  armature  poles  are  so 
wide  as  to  overlap,  when  in  the  position  shown  in  the  right-hand 
portion  of  Fig.  73,  the  wave  is  flattened  unduly  and  does  not, 
therefo?'e,  give  as  high  an  electromotive  force  as  could  otherwise 
be  obta'ned. 

The  effective  pressure  of  the  ordinary  magneto-generator,  when 
rotated  at  the  ordinary  speed  by  hand,  is  from  65  to  75  volts,  and 
it  may  be  made,  of  course,  higher  or  lower  to  meet  certain  re- 
quirements by  winding  with  a  greater  or  less  number  of  turns  or 
by  gearing  the  armature  so  as  to  rotate  with  greater  or  less 
speed. 

Some  telephone  linens,  as  for  instance  party  lines,  using  a  large 
number  of  instruments  in  series,  require  magneto-generators 


84  AMERICAN    TELEPHONE   PRACTICE. 

capable  of  producing  a  very  high  electromotive  force  in  order  to 
successfully  overcome  the  great  resistance  offered.  Inasmuch  as 
all  of  the  bells  are  in  series,  the  current  required  is  not  large.  In 
a  bridged  line,  however,  where  all  of  the  ringer  magnets  are  con- 
nected across  the  line  in  parallel,  the  current  required  is  heavy, 
while  the  voltage  need  not,  as  a  rule,  be  so  high.  In  long  lines 
of  this  latter  type  using  a  high-resistance  wire,  it  becomes  neces- 
sary to  develop  enough  pressure  to  overcome  the  resistance  of 
the  line  wire  in  order  to  ring  the  bell  at  the  farthest  end,  and  also 
a  sufficient  current  to  pass  in  multiple  through  all  of  the  ringers. 
In  this  case  a  rather  high  voltage  is  required  and  a  heavy  current, 
so  that  the  total  amount  of  energy  is  large  and  cannot  be  effect- 
ive merely  by  winding  the  instrument  to  a  higher  resistance.  In 
generators  of  this  type  it  is  customary  to  use  heavy  and  very 
powerful  permanent  magnets  and  to  exercise  the  greatest  care  in 
the  construction  to  produce  the  highest  efficiency. 

The  construction  of  the  polarized  call-bell,  or  ringer,  is  a  mat- 
ter requiring  no  less  attention  to  detail  than  that  of  producing  an 
efficient  generator.  The  old  form  of  ringers,  using  a  cast-iron 
frame  polarized  by  small  electromagnets,  was  subject  to  very 
grave  defects.  The  frame  became  readily  polarized  in  one  di- 
rection or  the  other,  due  to  the  passage  of  a  heavy  current 
through  the  magnets,  and  would  thus  give  the  armature  a  set  to 
one  side  or  the  other,  which  frequently  succeeding  currents  of  a 
weaker  nature  could  not  overcome.  This,  with  the  fact  that 
with  every  reversal  of  the  current  the  entire  magnetic  field  set 
up  through  this  heavy  mass  of  poor-quality  iron  had  to  be  com- 
pletely reversed,  was  a  point  rendering  the  construction  of  an 
•efficient  ringer  almost  an  impossibility.  The  tendency  in  the 
present  form  of  ringers  is  to  make  a  magnetic  circuit  which  is 
subjected  to  the  changes  due  to  the  magnetizing  force  as  short 
as  possible  and  to  make  the  magnetic  circuit  of  the  very  best  pos- 
sible material.  Swedish  or  Norway  iron,  cold  drawn  and  an- 
nealed, has  been  found  to  meet  these  requirements  most  per- 
fectly. The  sticking  of  the  armature  to  one  pole  or  the  other  is 
further  prevented  by  the  interposition  of  a  thin  sheet  of  non- 
magnetic material,  usually  copper,  between  the  faces  of  the  arma- 
ture and  the  pole-pieces.  Sometimes  this  is  accomplished  by 
inserting  a  small  rivet  either  into  the  center  of  the  pole-piece  or 
into  the  armature  face  itself. 

The  length  of  the  rod  carrying  the  hammer  plays  a  consider- 
able part  in  the  sensitiveness  of  the  bell.  A  long  rod  will  secure 
for  the  hammer  a  long,  and  therefore  powerful,  stroke,  but  the 


CALLING   APPARATUS.  85 

sensitiveness  is  correspondingly  reduced.  On  the  other  hand,  a 
short  rod  will  produce  a  short  and  comparatively  weak  stroke,  but 
the  bell  will  be  more  sensitive  than  with  the  long  rod. 

Other  points  in  the  design  of  magneto-generators  and  ringers 
will  be  taken  up  in  a  subsequent  chapter  on  Commercial  Forms 
of  Magneto-Bells.  Before  considering  these  in  detail,  however, 
certain  other  accessories  must  be  described. 


CHAPTER  VIII. 

THE   AUTOMATIC    SHUNT. 

ON  account  of  the  high  resistance  of  the  generator  armature 
and  its  great  retarding  effects,  it  is  desirable  to  have  it  shunted 
out  of  the  line  when  the  generator  is  not  in  use.  Especially  is 
this  desirable  on  party  lines  where  two  or  more  instruments  arc 
used  on  a  single  line.  To  accomplish  this  many  devices  have 
been  used,  both  automatic  and  manual.  The  automatic  devices 
have  now  almost  entirely  supplanted  the  manual,  as  the  latter 
were  never  satisfactory,  owing  to  the  inability  of  ignorant  and 
careless  persons  to  properly  manipulate  them.  Many  styles  of 
these  automatic  shunting  devices  have  come  into  general  use,  the 
ones  shown  and  described  being  typical.  Referring  to  Fig.  74, 


Fig.  74. — Western  Electric  Armature  Shunt. 

which  shows  the  shunt  used  by  the  Bell  Company,  the  gear-wheel, 
G,  is  mounted  on  the  crank-shaft,  S,  and  is  free  to  turn  thereon 
through  a  small  portion  of  a  revolution.  Terminals,  a  a',  are 
connected  to  the  terminals  of  the  armature  winding. 

When  the  generator  is  at  rest  a  current  coming  over  the  line 
will  pass  from  a  through  the  crank-shaft  and  out  through  the 
spring,  o,  to  the  terminal,  a;  this  path  being  of  almost  no  resist- 
ance, while  that  of  the  armature  winding  is  large.  When  the 
crank  is  turned,  however,  the  pin,/,  rides  out  of  the  notch  in  the 
hub  of  the  gear-wheel  and  in  so  doing  pulls  the  shaft  out  of  con- 
tact with  its  spring,  o,  thus  breaking  the  low  resistance  path  or 
shunt  around  the  armature,  and  leaving  the  latter  effectively  in 
the  line. 

86 


THE  AUTOMATIC  SHUNT.  87 

In  Fig.  75,  A  is  the  core  of  the  armature,  G'  its  pinion,  and  w 
a  diagrammatic  representation  of  the  winding.  While  at  rest 
current  from  the  line,  instead  of  passing  through  the  coil,  w,  will 
take  the  path  .  from  #' through  the  core,  A,  to  the  spring,  5, 
thence  to  pin,/, on  which  5  normally  rests  and  thence  out  through 
pin,  c}  to  the  terminal,  a.  When^he  armature  is  revolved  the  cen- 


Fig.  75- — Centrifugal  Armature  Shunt. 

trifugal  force  of  the  end  of  spring,  5,  causes  contact  to  be  broken 
between  it  and  pin,  /,  which  opens  the  shunt  around  the  arma- 
ture winding. 

In  Fig.  76,  G  is  the  large  gear-wheel  and  G'  its  pinion  on  the 
armature  shaft.  The  low-resistance  path  around  the  armature  is 
from  point  a  through  spring,  5',  screwed  on  the  inside  of  the 
generator  box,  through  spring,  5,  gear,  G,  to  the  frame  of  the 
machine  and  out  at  a '.  When  the  crank  is  turned,  the  collar,  c, 


Fig.  76.— Western  Telephone  Construction  Co.  Armature  Shunt. 

which  is  loose  on  the  shaft,  z,  but  rigid  with  the  crank,  forces 
the  spring,  5,  away  from  the  spring,  5',  by  virtue  of  the  pin,  py 
mounted  on  the  shaft,  z,  engaging  the  spiral  slot  in  the  collar,  c. 


88 


AMERICAN   TELEPHONE  PRACTICE. 


In  the  later  forms  of  this  shunt,  which  is  that  of  the  Western 
Telephone  Construction  Company,  a  disk  collar  pressed  from 
sheet  brass  is  interposed  between  the  springs,  5  and  5',  thus 
affording  a  better  contact  surface  for  the  spring,  S'. 

A  shunt  device  recently  put  on  the  market  by  the  Sterling 
Electric  Company  is  shown  in  Fig.  77.  The  shunt  mechanism  is 
operated  by  the  crank-shaft,  which  carries  the  large  gear-wheel. 
This  shaft  turns  in  a  hollow  sleeve,  24,  which  is  journaled  in  the 
brackets,  25,  mounted  on  the  end  plates  of  the  machine.  A  pin, 
27,  fixed  in  the  crank-shaft,  23,  engages  a  diagonal  slot,  26,  in 
the  sleeve,  24.  This  pin  is  held  at  one  end  of  the  slot  by  the 
spring,  28,  which  is  coiled  around  and  fastened  to  the  sleeve,  24. 


Fig.  77- — Details  of  Cook  Armature  Shunt. 

When,  however,  the  crank-shaft  is  rotated,  the  pin,  27,  rides  against 
the  side  of  the  slot,  26,  until  it  assumes  the  position  shown  in  the 
upper  right-hand  portion  of  the  figure,  after  which  the  sleeve  and 
gear  turn  with  the  shaft.  This  causes  the  crank-shaft,  23,  to  break 
contact  with  spring,  29,  and  thus  break  the  shunt  around  the 
generator. 

Another  shunt,  dependent  on  an  entirely  different  principle 
very  ingeniously  applied,  is  that  used  by  the  Holtzer-Cabot  Com- 
pany. A  small  cylindrical  case  of  brass  is  mounted  directly  on  the 
projecting  portion  of  .the  armature  shaft,  and  is  in  metallic  contact 
therewith.  The  insulating  pin  with  which  the  other  terminal  of 
the  armature  winding  is  connected  projects  into  the  chamber 
formed  by  this  case  ;  but  does  not  come  into  metallic  contact  with 
the  casing  nor  the  armature  shaft  itself.  The  chamber  is  then 


THE   AUTOMATIC   SHUNT. 


89 


partially  filled  with  small  bits  of  metallic  wire  which  normally 
form  a  connection  between  the  central  pin  and  the  casing  itself, 
thereby  forming  a  short  circuit  or  shunt  around  the  armature 
winding.  When,  however,  the  armature  is  rotated,  the  centrif- 
ugal force,  due  to  the  rotation  acting  upon  the  bits  of  wire,  causes 
them  to  fly  to  the  outer  portion  of  the  casing,  thus  breaking  the 
contact  with  the  central  pin  and  removing  the  shunt  from  the 
armature.  This  is  probably  the^  simplest  shunt  on  the  market, 
and  should  prove  reliable.  In  order  to  prevent  corrosion  of  the 
parts,  the  casing  and  the  small  particles  of  wire  are  silver-plated. 

Still  another  shunt  of  new  design  is  that  used  by  the  Williams 
Electric  Company  on  their  new  generators.  It  is  shown  in 
Fig.  78. 

In  this  figure  the  crank-shaft,  s,  is  tubular  and  incloses  the  shaft, 
s't  on  which  the  gear-wheel  is  mounted.  The  crank-shaft  is 


Fig.  78. — Williams  Armature  Shunt. 

capable  of  a  slight  rotation  before  the  gear-shaft  is  moved,  such  ro- 
tation, or  what  may  be  termed  lost  motion,  being  consumed  in  de- 
pressing two  springs,  s  s',  which  actuate  levers,  pp',  within  the  cup- 
shaped  piece,  c,  mounted  on  the  crank  shaft.  One  or  both  of  these 
levers  makes  contact  with  the  arched  metallic  strip,  A,  insulated 
from  the  frame  of  the  generator,  while  the  crank  is  at  rest.  As 
soon,  however,  as  it  is  turned  the  lost  motion  is  taken  up  in  remov- 
ing the  levers  from  contact  with  this  arch.  This  breaks  the  shunt 
which  is  formed  from  the  frame  of  the  machine,  which  is  in  contact 
with  one  terminal  of  the  armature  winding,  through  the  levers 
to  the  arched  strip,  which  is  connected  by  a  wire,  to  the  spring 
at  the  right-hand  portion  of  the  generator,  which  bears  upon 
the  armature  spindle. 


CHAPTER    IX. 

THE    HOOK   SWITCH   AND    CIRCUITS   OF   A   TELEPHONE. 

So  far  we  have  considered  the  talking  apparatus  and  the  call- 
ing apparatus  separately.  It  is  obvious  that  inasmuch  as  these 
are  used  alternately,  some  means  is  necessary  for  switching  one 
or  the  other  into  the  circuit.  As  an  instrument  must,  when  not 
in  use,  be  ready  to  respond  to  a  call,  the  call-bell  must,  of  neces- 
sity, be  normally  left  in  the  line  ;  and  further,  as  the  resistance 
and  self-inductance  of  the  call-bell  magnets  would  be  detrimental 
to  the  transmission  of  talking  currents,  the  call-bell  must  in  most 
cases  be  switched  out  of  the  line  when  the  talking  instruments 
are  in  use. 

At  first,  hand  switches  were  used  to  accomplish  this  result,  and 
even  before  the  adoption  of  the  battery  transmitter  the  instru- 
ments were  provided  with  ordinary  two-point  hand  switches,  so 
arranged  as  to  alternately  close  the  line  circuit  through  two 
branches — one  containing  a  call-bell  and  generator,  and  the 
other  the  magneto-telephone.  It  was  soon  found  necessary  to 
make  this  switch  as  nearly  automatic  as  possible,  as  careless  or 
ignorant  users  would  frequently  leave  it  in  the  wrong  position. 
To  attain  this  end,  the  switch  lever  was  so  designed  as  to  be 
held  by  the  weight  of  the  receiver  in  contact  with  a  terminal  of 
the  calling  circuit,  but  when  released  therefrom  to  be  moved  by 
a  spring  into  contact  with  the  talking  circuit  terminal.  Soon 
after,  battery  transmitters  having  come  into  general  use,  it 
became  necessary  to  provide  means  for  opening  and  closing  a 
local  circuit  containing  the  local  battery,  the  primary  of  the 
induction  coil,  and  the  microphone  transmitter.  This  was  done 
in  order  to  have  the  battery  in  use  only  when  the  telephone 
instrument  was  being  used,  and  was  accomplished  by  the  addition 
of  a  single  contact  point  with  which  the  hook  made  contact 
when  released  from  the  weight  of  the  receiver. 

Fig.  79  shows  the  circuit  of  an  ordinary  telephone  instrument. 
The  hook,  H,  is  shown  in  its  depressed  position  as  though  under 
the  weight  of  the  receiver.  In  this  position  all  talking  circuits 
are  inoperative,  being  open  at  the  points,  I  and  2,  and  are  for  that 
reason  represented  by  dotted  lines.  A  calling  current  from  some 

go 


THE  HOOK  SWITCH  AND   CIRCUITS  OF  A    TELEPHONE.        91 

other  station  coming  over  line  wire,  Z,  would  pass  through  wire, 
a,  to  the  generator,  G,  thence  through  the  windings  of  the  call- 
bell  magnet,  C,  to  the  contact  point,  3,  through  the  lever  of  the 
hook  switch  and  out  through  line  wire,  L ',  or  to  ground,  in  case 


r F 


Fig.  79. — Telephone  Circuits,  Hook  Down. 

no  return  wire  is  used.  This  current  will  ring  the  bell.  To  ob- 
viate the  necessity  of  this  current  passing  through  the  armature 
winding  of  the  generator,  a  shunt  should  be  provided,  as  described 
in  the  last  chapter.  When  the  instrument  is  used  for  sending  a 
call  the  crank  of  the  generator  is  turned,  automatically  breaking 
the  shunt  around  the  armature  and  sending  the  current  out  over 


Fig.  80. — Telephone  Circuits,  Hook  Up. 

the  line  through  the  call-bell  magnets  of  this  instrument  to  those 
of  the  distant  station. 

In  Fig.  80  the  hook  is  shown  in  its  raised  position,  as  when 
released  from  the  weight  of  the  receiver.  The  circuit  through 
the  generator  and  call-bell  is  inoperative,  being  open  at  the 


92  AMERICAN   TELEPHONE  PRACTICE. 

point,  3,  and  is  therefore  shown  dotted.  The  local  circuit  con- 
taining the  primary  winding,  P,  of  the  induction  coil,  the  battery, 
B,  and  the  transmitter,  T,  is  closed  by  the  switch  lever  making 
contact  with  the  points,  I  and  2.  Current  therefore  flows  in  this 
circuit,  and  variations  in  the  resistance  of  the  microphone,  T, 
cause  corresponding  variations  in  this  current,  which  induce  cur- 
rents in  the  secondary  winding,  S,  of  the  induction  coil.  These 
currents  pass  from  the  secondary  coil  to  the  point,  2,  thence 
through  the  switch  lever  to  line,  Lr,  to  the  instrument  at  the  other 
end  of  the  line,  back  by  line,  Z,  and  through  the  winding  of  the 
receiver,  R,  to  the  secondary  coil,  5.  An  incoming  current  from 


Fig.  81.  —  Telephone  Circuits,  Hook  Down. 


a  distant  station  follows  the  same  path,  and  causes  the  diaphragm 
of  the  receiver  to  reproduce  sound. 

In  Figs.  8  1  and  82  are  shown  circuits  and  apparatus  for  accom- 
plishing the  same  results,  but  in  a  slightly  different  way.  It  will 
be  seen  that  the  circuit  through  the  generator  and  call-bell,  and 
that  through  the  receiver  and  secondary  winding,  are  perma- 
nently closed,  and  are  unaffected  as  to  their  continuity  by  the 
position  of  the  hook  switch.  In  Fig.  81  the  hook  is  depressed, 
thus  rendering  operative  the  calling  apparatus.  The  circuit 
through  the  instrument  is  now  from  line,  L,  to  the  generator,  G, 
thence  through  the  call-bell  magnets,  C,  through  the  switch  lever 
to  the  point,  3,  and  by  way  of  wire,  <z,  to  the  line  wire,  L'.  A 
current  from  the  generator,  G,  of  this  station  or  another  would 
pass  through  the  secondary  coil,  5,  and  the  receiver,  R,  were  it 
not  for  the  fact  that  the  wire,  a,  affords  a  path  of  practically  no 
resistance,  thus  short-circuiting  the  receiver  and  secondary. 

In  Fig.  82  the  hook  switch  is  in  the  talking  position  and  the 


THE  HOOK  SWITCH  AND   CIRCUITS  OF  A    TELEPHONE.        93 

generator,  G,  and  the  call-bell,  C,  are  rendered  inoperative  by 
virtue  of  the  low-resistance  path,  b,  being  closed  around  them. 
The  circuit  through  the  instrument  is  now  through  the  wire,  b, 
contact  point,  i,  lever,  H,  secondary  coil,  5,  and  receiver,  R.  In 
the  arrangement  shown  in  Figs.  81  and  82  the  local  circuit  is 
operated  in  the  same  manner  as  that  shown  in  Figs.  79  and  80. 
The  practice  of  leaving  both, the  calling  and  the  talking  circuits 
permanently  closed  and  of  shunting  one  or  the  other  out  of  the 
circuit  is  a  good  one,  for  if  the  hook  does  not  make  proper  con- 
tacts the  apparatus  is  still  operative,  although  its  efficiency  is  im- 
paired. 

Although  the  automatic  switching  apparatus  is  very  simple, 
much  care  is  necessary  in  its  design  and  construction.  The 
energy  available  for  the  operation  of  the  switch  is  limited  to  that 
due  to  the  attraction  of  gravity  on  the  receiver,  and  it  becomes 
somewhat  difficult  to  so  arrange  the  contacts  that  they  will  be 


Lint 


Fig.  82. — Telephone  Circuits,  Hook  Up. 

firmly  and  positively  made,  and  surely  broken  at  the  proper  time. 
For  this  reason  all  the  points  of  contact  are  preferably  provided 
with  platinum  tips  to  prevent  corrosion,  and,  if  possible,  a  slight 
sliding  action  at  the  point  of  contact  should  be  obtained.  A 
sliding  contact  tends  to  clean  the  points  and  at  the  same  time 
prevents  particles  of  dust  from  keeping  the  two  apart.  Too 
much  sliding  action  is,  however,  worse  than  none,  as  it  is  sure  to 
cause  cutting.  The  springs  for  restoring  the  lever  and  those 
serving  as  contacts  should  be  so  arranged  that  no  movement  of 
which  the  lever  is  capable  will  strain  them  beyond  their  elastic 
limit  or  to  such  a  degree  that  they  will  eventually  lose  their  ten- 
sion or  break.  It  is  bad  practice  to  have  the  same  part  of  a  con- 
tact slide  alternately  over  a  conducting  material,  as  of  brass,  and 


94 


AMERICAN   TELEPHONE  PRACTICE. 


an  insulating  material,  as  of  hard  rubber,  as  small  particles  from 
either  surface  are  sure  to  be  carried  upon  the  other  surface,  thus 
forming  a  partial  electrical  connection  on  the  insulating  surface 
and  a  defective  connection  on  the  brass  or  metal  surface. 
Where  a  sliding  contact  is  used  much  trouble  is  often  caused  by 
the  cutting  of  the  two  surfaces.  The  extent  of  this  cutting, 
even  where  the  pressure  is  very  light  and  the  movement  very 
limited,  is  often  astonishing. 

In  Fig.  83  is  shown  the  hook  switch  now  almost  universally 
used  by  the  Bell  Telephone  Company,  and  known  as  the 
"  Warner  Switch."  The  hook  lever  is  pivoted  to  a  bracket  by 
a  screw  as  shown,  and  is  provided  with  a  lug,/,  and  a  strip  of 
insulating  material,  g,  on  its  short  arm.  On  the  under  side  of 
the  lever  is  an  insulating  pin,  h,  and  a  contact  point,  i.  A  spring 
screwed  to  the  generator  box  under  the  lever  by  the  screw,  b, 


Fig.  83.— Warner  Hook  Switch. 

bears  alternately  upon  the  insulating  pin,  h,  and  the  contact 
point,  i,  and  tends  to  press  the  lever  into  its  elevated  position. 
Springs,  c  and  d,  screwed  to  the  side  of  the  generator  box  bear 
alternately  upon  the  insulating  piece,  g,  and  the  conducting  lug, 
fj  according  to  whether  the  lever  is  depressed  or  elevated.  The 
spring,  c,  is  connected  through  the  secondary  winding  of  the  in. 
d  action  coil  and  the  receiver  to  one  side  of  the  line.  The  screw, 
bj  is  connected  through  the  calling  apparatus  to  the  same  side  of 
the  line.  The  binding  screw,  a,  connected  with  the  lever,  ^, 
forms  the  terminal  of  the  other  side  of  the  line.  The  local 
circuit  terminates  on  one  side  in  the  spring,  c,  and  on  the  other 
side  in  the  spring,  d.  When  the  hook  is  depressed,  point  i  is 
connected  through  the  lower  spring  with  the  screw,  b,  and  the 


THE   HOOK  SWITCH  AND   CIRCUITS  OF  A    TELEPHONE.        95 

calling  circuit  is  complete.  Both  the  local  circuit  and  the  line 
circuit  through  the  talking  apparatus  are  broken  at  springs,  c  and 
d.  When  the  hook  is  elevated,  the  calling  circuit  is  broken  at 
the  point  i,  and  the  local  and  line  circuits  are  completed  by  the 
springs,  c  and  d,  and  the  lug,/. 

This  hook  switch  is  as  perfect  as  any  on  the  market,  and  a 
study  of  it  is  interesting  as  showing  a  nicety  of  detail  that  can  be 
appreciated  only  by  those  wlio  have  had  practical  experience  in 
telephony. 

When  platinum  is  not  used  on  hook-switch  contacts,  it  is  a 
matter  of  absolute  necessity  to  have  rubbing  contacts,  and  cut- 
ting may  be  reduced  to  a  minimum  by  making  the  contact  sur- 
faces of  dissimilar  metals.  German-silver  springs  bearing  on 
brass  contacts  form  as  good  a  combination  as  can  be  obtained 
without  the  use  of  platinum.  The  lever  of  the  hook  nearly 


Fig.  84. — Short  Lever  Hook  Switch  with  Poor  Contacts. 


always  forms  one  branch  of  the  circuit,  and  in  no  case  should  the 
contact  through  the  pivot  screw  be  depended  on  for  conduc- 
tivity. A  good  plan  is  to  form  a  soldered  connection  between 
the  lever  and  base,  by  means  of  a  short  spiral  of  flexible  wire, 
soldered  at  one  end  to  the  lever  and  at  the  other  to  the  base, 
the  connection  being  made  at  points  where  the  relative  motion 
between  the  two  parts  is  a  minimum. 

Fig.  84  shows  a  common  type  of  switch  which  should  be 
guarded  against.  The  short  contact  springs  have  little  flexibility, 
and  cutting  between  them  and  the  hook  lever  soon  sets  in.  The 
friction  soon  becomes  so  great  that  the  lever  will  stick  in  one 
position  or  the  other,  and  neither  the  weight  of  the  receiver  nor 
the  strength  of  the  retractile  spring  is  sufficient  to  move  it. 

Figs.  85  and  86  show  two  hook  switches  manufactured  by 
the  Holtzer-Cabot  Electric  Company.  The  former  is  adapted 
for  mounting  in  the  bottom  of  the  generator  box,  in  front  of  the 
generator,  and  the  latter  on  the  side  of  the  box,  above  the 
generator.  These  hooks  have  the  advantage  of  being  self-con- 


96 


AMERICAN    TELEPHONE   PRACTICE. 


tained,  and  depend  on  a  rather  heavy  sliding  contact  obtained  by 
the  use  of  long  flexible  springs. 

The  diagrams  of  circuits  shown  in  Figs.  79,  80,  81,  and  82  were 


Fig.  85  and  86. — Long  and  Short  Lever  Hook  Switches, 
somewhat  simplified   in  order  to  render  clearer  the  circuits. 


In 


Fig.  87  is  shown  the  circuits  of  a  complete  telephone  set,  the 
connections  being  arranged  as  in  practice.  It  is  customary  to 
mount  the  generator,  G,  polarized  bell,  P,  and  hook  switch  all  in 
one  box.  In  order  to  facilitate  the  work  of  making  connections 


Fig.  87. — Complete  Circuits  of  Series  Telephone. 

between  the  parts  contained  in  the  generator  box  and  the  other 
parts,  i.  e.,  the  transmitter,  induction  coil,  receiver,  and  batteries, 
the  terminals  of  all  the  circuits  in  the  box  are  brought  out  on 


THE  HOOK  SWITCH  AND   CIRCUITS   OF  A    TELEPHONE.        97 

binding  posts  on  the  top  and  bottom  of  the  box.  The  binding 
posts,  I  and  2,  on  top  of  the  box  are  for  attaching  the  line 
wires.  These  form  the  terminals  of  the  instrument  as  a  whole. 
The  center  binding  post  forms  the  ground  terminal  of  the  light- 
ning arrester,  and  has  no  connection  within  the  box. 

The  six  binding  posts  at  the  bottom  of  the  generator  box  are 
in  three  pairs,  r,  /,  and  s.  The  pair,  r,  form  terminals  for  attach- 
ing the  receiver  cord.  The  two  posts  forming  pair,  /,  are  for  the 
local  circuit,  containing  between  these  posts  the  battery,  B,  the 
transmitter,  T,  and  the  primary  of  the  induction  coil,  /.  Be- 
tween the  right-hand  pair  of  posts,  s,  is  connected  the  secondary 
of  the  induction  coil,  /.  The  bell,  P,  is  mounted  on  the  door  of 
the  box,  connection  to  it  being  made  through  the  hinges. 

The  connections  of  the  automatic  shunt  are  clearly  shown  in 
this  figure.  Normally  a  short  circuit  exists  around  the  genera- 
tor, through  the  wire,  S,  spring,  $,  disk,  c,  and  the  frame  of  the 
machine.  This  is  broken  between  the  spring,  b,  and  disk,  c, 
when  the  generator  is  operated  as  already  described. 

Fig.  88  is  a  view  of  a  complete  instrument,  showing  quite 
clearly  the  external  connections  of  the  generator  box.  In  this, 
posts,  E  and  C,  are  the  line  terminals,  and  D  the  ground  post 
for  the  lightning  arrester.  The  two  left-hand  binding  posts  on 
the  bottom  of  the  box  form  terminals  for  the  receiver  cord,  as 
shown.  The  two  center  posts,/  and  //,  correspond  with  the  pair 
of  posts, /,  in  Fig.  87.  Thecurled  wire  from  /passes  through  the 
back  board  of  the  instrument  and  to  one  terminal  of  the  battery 
in  the  battery  box  below.  The  post,  //,  is  connected  by  a  similar 
wire  to  the  binding  post,  N,  on  the  transmitter  base,  from 
which  the  circuit  may  be  traced  through  the  primary  of  the  in- 
duction coil,  thence  to  the  metallic  portion  of  the  transmitter 
arm  and  base  to  the  carbon  electrode  of  the  transmitter,  then 
through  the  transmitter  and  -back  by  a  flexible  cord  along  the 
side  of  the  arm,  which  connects  with  the  lower  post,  Z,  on  the 
base.  This  post  is  then  connected  by  a  curled  wire  passing 
through  the  back  board  to  the  remaining  terminal  of  the 
battery. 

This  instrument,  which  is  made  by  the  Western  Telephone 
Construction  Company,  possesses  a  unique  form  of  switch 
hook.  The  hook  lever  is  journaled  in  the  side  of  the  box,  and  is 
provided  with  a  short,  downwardly  bent  lever,  carrying  on  its 
inner  end  a  small  anti-friction  shoe.  Against  this  shoe  bears  the 
long  heavy  spring,  O,  screwed  to  the  inside  of  the  generator  box. 
The  strength  of  this  spring,  which  presses  toward  the  left,  is 


98 


AMERICAN   TELEPHONE   PRACTICE. 


sufficient  to  keep  the  hook  elevated  while  it  is  not  supporting  the 
receiver.  The  weight  of  the  receiver,  however,  forces  the  short 
arm  of  the; lever  toward  the  right,  moving  the  spring,  O,  with  it. 


Fig.  88. —  Complete  Telephone  Instrument. 

The  changes  of  circuit  are  accomplished  by  the  movements  of 
thte  spring,  and  not  by  the  lever  itself,  which  forms  no  part  of  the 
circuit.  The  contact  springs  with  which  this  main  spring 
engages  are  long  and  flexible,  and  are  mounted  horizontally  on 


THE   HOOK  SWITCH  AND   CIRCUITS   OF  A    TELEPHONE. 


99 


the  inside  of  the  box.  These  springs  are  platinum-pointed,  as  is 
also  the  main  spring,  so  that  the  contacts  are  made  and  broken  in 
a  reliable  manner.  The  old  forms  of  this  hook  switch  were  very 
faulty.  The  anti-friction  shoe,  and  the  platinum  contacts,  how- 
ever, together  with  the  improved  design  of  the  springs,  have  ren- 
dered it  thoroughly  reliable. 

The  details  of  the  generator  and  ringer  of  this  instrument  will 
be  described  in  the  next  chapter.  fc  Another  feature  of  interest  in 
this  set  is  the  small  coil,  A,  not  shown  in  the  diagram  of  circuits. 
This  also  will  be  discussed  in  a  subsequent  chapter. 

The  circuits  of  a  bridging-bell  instrument  are  shown  in  Fig.  89. 


fi" 


i— « 


Fig.  89. — Complete  Circuits  of  Bridged  Telephone. 

This  instrument  is  especially  adapted  to  use  on  party  lines,  but  is 
also  used  to  a  large  extent  in  general  exchange  work. 

In  this  instrument  the  call-bell,  P,  is  permanently  bridged  across 
the  two  sides  of  the  line  between  the  binding  p.osts,  I  and  2,  and 
its  magnets  are  made  of  high  resistance  and  retardation.  A  little 
consideration  will  show  that  the  bell  circuit  is  not  affected  by  the 
position  of  the  hook  lever,  there  being  no  lower  contact  to  the 
switch.  The  generator,  G,  is  in  a  second  bridge  circuit,  which  is 
normally  open,  but  closed  when  the  generator  is  operated.  The 
talking  circuit,  containing  the  receiver,  R,  and  secondary  winding  of 


100 


AMERICAN  TELEPHONE   PRACTICE. 


the  induction  coil,  /,  forms  a  third  bridge  circuit,  which,  like  the 
generator  circuit,  is  normally  open. 

The  telephone  circuit  is  automatically  closed  when  the  receiver 
is  removed  from  its  hook  for  use,  and  this  operation  also  closes 
the  local  circuit,  containing  the  primary  of  the  induction  coil,  /, 
the  local  battery,  B,  and  the  transmitter,  T.  The  talking  circuits 
are  identical  with  those  in  Fig.  87.  In  order  that  there  shall 
not  be  an  undue  leakage  of  the  voice  currents  through  the 
permanently  bridged  call-bell  circuit,  the  magnets  of  these  call- 
bells  are  wound  to  a  high  resistance  (usually  a  thousand  ohms) 
and  are  also  constructed  in  such  manner  that  they  will  have 
a  high  coefficient  of  self-induction. 

The  closing  of  the  generator  bridge  upon  the  sending  of  a  call 
may  be  accomplished  manually,  as  with  the  key,  k.  It  is  usually 


•in 


•F 


Fig.  90. — Circuits  of  Battery  Call  Instruments. 


done  automatically,  however,  by  a  device  similar  to  the  automatic 
shunt  used  in  the  regular  instruments.  Thus,  if  the  wire  leading 
from  binding  post,  2,  in  Fig.  89,  were  led  to  the  spring,  b,  instead 
of  to  the  frame  of  the  generator,  and  the  binding  post,  I,  per- 
manently connected  to  the  armature  spindle  pin,  it  is  evident 
that  the  inward  movement  of  the  disk,  c,  caused  by  turning  the 
generator  crank,  would  accomplish  the  same  result  as  pressure  on 
the  key,  k,  and  with  the  advantage  of  not  requiring  the  volition  of 
the  operator. 

The  specific  arrangement  of  circuits  shown  in  Fig.  89  and  their 
use  in  multiple  on  party  lines  is  patented  by  Mr.  John  J.  Carty, 
engineer  of  the  New  York  Telephone  Company.  The  operation 


THE   HOOK  SWITCH  AND    CIRCUITS  OF  A    TELEPHONE.      101 

and  design  of  this  instrument  will  be  considered  at  greater  length 
under  the  head  of  Party  Lines. 

The  circuits  of  a  battery  call  set  are  shown  in  Fig.  90.  In  this 
I  and  2  are  the  line  binding  posts,  and  3  a  post  forming  one  ter- 
minal of  the  local  battery,  the  post,  I,  also  serving  as  a  battery 
terminal.  When  the  hook,  L,  is  depressed  by  the  receiver  the 
circuit  passes  from  line  post,'  I,  through  the  hook  lever,  back  con- 
tact of  push-button,  P,  vibrating  call-bell,  V,  and  to  line  post,  2. 
The  instrument  is  therefore  ready  to  receive  a  call.  To  send  a  call, 
battery,  B,  is  connected  between  the  line  posts  by  pressing  the 
button,  P ;  the  circuit  being  traced  from  post,  I,  through  battery, 
/>,  and  the  two  upper  contacts  of  the  button,  to  binding  post,  2. 
The  talking  circuits  are  closed  in  the  ordinary  manner  by  the 
raising  of  the  hook  lever. 

Fig.  91    shows   a   type    of  telephone    set    which   is   becoming 


Fig.  91. — Desk  Telephone  Set. 

very  popular  for  business  men  who  do  not  desire  to  leave  their 
desks  in  order  to  use  the  telephone.  The  particular  set  illus- 
trated is  made  by  the  Holtzer-Cabot  Company.  The  transmitter 
is  of  the  granular-carbon  type  mounted  on  a  handsome,  nickel- 
plated  and  polished  wood  stand,  as  shown.  One  terminal  of  the 
transmitter  is  formed  by  the  frame  itself,  while  the  other  terminal 
is  carried  down  the  inside  of  the  tube,  or  standard.  The  lever  of 


102 


AMERICAN   TELEPHONE   PRACTICE. 


the  hook  switch  is  pivoted  at  its  end  in  an  enlargement  of  the 
standard  and  actuates  a  slender  rod  which  passes  down  into  the 
base,  where  it  engages  the  switch  lever.  This  lever  is  pivoted  on 
a  separate  bar,  mounted  on  a  fiber  block  and  acted  upon  by  a 
spiral  spring  in  such  a  manner  as  to  press  upward  against  the 
vertical  rod,  thus  tending  to  raise  the  hook.  In  this  position  a 
knife  edge,  carried  on  the  switch  lever  in  the  base,  presses  against 
the  two  springs,  which  form,  respectively,  terminals  of  the  pri- 
mary and  secondary  talking  circuits.  When  the  hook  is  sub- 
jected to  the  weight  of  the  receiver  the  switch  lever  is  depressed 
by  the  rod  against  the  tension  of  the  spiral  spring,  thus  breaking 
connection  with  the  two  springs  of  the  talking  circuit  and  making 
connection  with  a  spring  forming  the  terminal  of  the  calling  cir- 
cuit. The  induction  coil  is  mounted  in  the  base.  This  apparatus 


Fig.  92. — Circuits  of  Comple  Desk  Set. 

may  be  used  in  connection  with  a  magneto-generator  and  call- 
bell,  which  are  usually  placed  under  the  desk  or  at  some  point 
where  the  generator  crank  is  within  easy  reach  of  the  user,  or  in 
connection  with  a  battery-call  outfit,  in  which  case  the  circuits 
would  be  similar  to  those  shown  in  Fig.  90. 

Fig.  92  shows  the  circuits  of  an  apparatus  similar  to  this,  manu- 
factured by  the  Western  Telephone  Construction  Company. 
Seven  binding  posts  are  arranged  in  this  set  cfn  the  upper  side  of 
the  magneto-box,  to  which  all  terminals  from  the  various  pieces 
of  apparatus  are  run.  i  and  2  form  the  line  binding  posts,  and  6 
and  7  the  battery  binding  posts,  the  other  terminals  being  con- 


THE   HOOK  SWITCH  AND   CIRCUITS  OF  A    TELEPHONE.      103 

nected  as  shown.  Inasmuch  as  the  induction  coil  in  this  set 
is  mounted  in  the  generator  box,  it  becomes  necessary  for  five 
different  conductors  to  run  from  the  generator  box  to  the  desk 
stand.  These  conductors  are  constructed  in  the  same  manner  as 
an  ordinary  receiver  cord,  there  being  five  strands  instead  of  two. 
In  this  latter  set  the  hook  switch  is  of  the  same  type  as  previously 
described  in  connection  with  Fig.  88,  but  is  so  modified  as  to 
enable  it  to  be  placed  in  the  vertical  standard  supporting  the 
transmitter.  It  is  platinum-pointed  and,  as  recently  modified, 
should  prove  reliable. 

The  connections  in  a  telephone  cannot  be  too  carefully  made. 
All  possibility  of  two  wires  coming  into  accidental  contact  should 
be  carefully  avoided.  All  joints  should  be  soldered,  without  the 
use  of  acid.  Where  connection  is  made  under  the  head  of  a 
screw  passing  through  the  wood  of  the  box,  means  must  be  taken 
to  prevent  the  loosening  of  the  connection  due  to  shrinkage  of 
the  wood.  If  possible,  the  connection  should  be  soldered  ;  if 
not,  a  spring  washer  may  be  used. 


CHAPTER  X. 

COMMERCIAL    CALLING   APPARATUS. 

THE  combination  of  a  magneto-generator  and  a  polarized  bell 
or  ringer,  mounted  in  a  suitable  box,  is  usually  termed  a  magneto- 
bell.  The  hook  switch,  from  the  fact  that  it  is  usually  mounted 
in  the  generator  box,  is  often  counted  as  a  part  of  a  com- 
plete magneto-bell.  Owing  to  the  lively  competition  between 
various  manufacturers,  and  also  to  the  increasing  demands  for 
good  service  on  the  part  of  the  public,  great  improvement  has 
been  made  in  this  line  of  work  during  the  last  few  years.  This 
chapter  will  be  devoted  to  a  consideration  of  some  of  the  more 
approved  forms  of  this  very  important  part  of  telephone 
equipment. 

The  details  of  the  Western  Telephone  Construction  Company's 
large  generator  for  heavy  work  are  shown  in  Figs.  93,94,  and  95. 


Figs.  93  and  94. — Details  Western  Telephone  Construction  Co.  Generator. 

This  instrument  is  very  similar  to  the  one  used  by  the  Bell 
companies.  In  it  the  pole-pieces  are  of  cast  iron,  riveted  together 
by  means  of  the  shouldered  brass  rods,  B  B.  After  this  they  are 
bored  by  a  special  tool  to  the  required  internal  diameter  to  re- 
ceive the  armature.  The  core  of  the  latter  is  of  cast  iron  and  is 
shown  in  Fig.  95,  being  accurately  turned  to  fit  between  the  pole- 


COMMERCIAL    CALLING  APPARATUS.  105 

pieces.  The  bearing  plates  are  of  cast  brass  with  a  shoulder  also 
turned  to  fit  between  the  pole-pieces  so  as  to  be  self-centering  when 
secured  in  place.  They  are  each  fastened  to  the  ends  of  the  pole- 
pieces  by  four  screws,  as  shown.  The  gears  are  cut  from  heavy 
cast  brass,  the  large  gear  being  mounted  on  a  shaft  journaled  in 
the  same  bearing  plates  as  the  armature  itself. 

The  magnets  are  bent  cold  from  I"  x  I"  magnet  steel,  and  are 
secured  in  place  by  clamping  plates,  C  C,  and  screws,  5  5,  the 
latter  passing  between  the  magnets  and  into  the  pole-pieces. 
This  generator,  while  it  embodies  no  new  or  radical  features,  is 
well  made  and  generally  satisfactory.  It  would  give  better 
results,  however,  were  the  armature  of  smaller  diameter.  The  air 
gap  in  machines  of  this  type  may  be  reduced  to  less  than  1-J-Tr  of 
an  inch  without  endangering  the  smooth  running  of  the  arma- 
ture. The  automatic  shunt  already  referred  to  is  shown  to  better 
advantage  in  these  figures. 

The  polarized  bell  used  with  the  later  types  of  this  instrument 
is  shown  in  Fig.  88.  The  two  coils,  />,  are  parallel,  and  attached 


.  95- — Armature  Core  of  Generator. 

at  one  end  to  a  soft-iron  yoke,  the  ends  of  which  extend  beyond 
the  coils,  to  receive  the  two  round  bar-magnets,  which  polarize 
the  frame  of  the  bell. 

On  the  projecting  ends  of  these  two  permanent  magnets  is 
mounted  a  second  yoke  bar  of  soft  iron,  on  which  is  adjustably 
mounted  the  bracket  in  which  the  armature  is  pivoted.  The  two 
magnets  have  their  forward  ends  of  one  polarity  and  their  rear 
ends  of  the  other ;  and,  together  with  the  two  yokes,  form  a 
rectangle,  one  of  the  yokes  being  of  positive  polarity,  the  other 
negative.  In  this  rectangle  are  mounted  the  coils  and  armature 
of  the  ringer,  which  operate  in  the  usual  manner  by  the  alternating 
ringing  currents. 

The  Holtzer-Cabot  Electric  Company  are  manufacturing  an  ex- 
cellent magneto  set,  the  generator  and  ringer  of  which  are  shown 
in  Figs.  96  and  97.  The  end  plates  in  which  the  generator  arma- 
ture is  journaled  are  of  cast  brass  and  are  screwed  directly  to  the 
cast-iron  pole-pieces,  the  ends  of  which  are  flanged  and  machined 
so  as  to  fit  accurately.  The  armature  core  is  of  soft  sheet-iron 


io6 


AMERICAN    TELEPHONE  PRACTICE. 


laminations.  The  punchings  forming  the  core  are  clamped 
together  on  a  steel  rod,  which  therefore  serves  as  the  armature 
shaft.  The  drive-wheel  is  mounted  in  a  long  bearing,  adjustably 
secured  to  an  extension  on  the  right-hand  end  plate.  This  com- 
pany uses  two  forms  of  driving  gear,  one  the  regular  gear- 


Fig.  96. — Holtzer-Cabot  Generator. 

wheels  and  the  other  the  chain  and  sprocket  mechanism  shown. 
It  seems  to  prefer  the  latter,  which,  it  must  be  said,  when  provided 
with  a  steel  chain,  runs  very  easily  and  noiselessly.  This  genera- 
tor is  provided  with  the  centrifugal  shunt  described  in  Chapter 
VIII.,  mounted  directly  on  the  armature  shaft,  the  whole  forming 
a  very  efficient  combination.  The  generators  for  bridging-bell 
service  manufactured  by  this  company  are  of  a  similar  type,  but 
provided  with  four  magnets  instead  of  three,  and  a  correspond- 


-  97- — Holtzer-Cabot  Ringer. 

ingly  long  armature.     It  is  one  of  the  most  powerful  generators 
for  this  purpose  ever  tested  by  the  writer. 

The  ringer  furnished  with  this  generator  is  of  the  same  general 
type  illustrated  in  Fig.  72.  The  gongs  are  mounted  on  adjust- 
able standards  pivoted  at  their  upper  ends,  and  each  held  by 
a  screw  engaging  a  slot  in  their  lower  ends. 


COMMERCIAL   CALLING  APPARATUS. 


107 


The    Williams-Abbott    magneto-bell    is    shown    complete    in 
Fig.  98  and  in  detail  in  Figs.  99,  100,  and  101.     In  this  the  pole- 


Fig.  98. — Williams-Abbott  Magneto-Bell  Complete. 

pieces  of  the  generator  are  stamped  from  soft  sheet  iron  and 
fastened  to  the  circumferential  edges  of  the  end  plates  by  four 
screws  at  each  end.  The  edges  of  the  end  plates  are  of  the 
proper  curvature  to  maintain  the  inner  surfaces  of  the  pole-pieces 
in  their  proper  relation  to  the  armature.  The  lower  edges  of  the 


Figs.  99  and  ioo,-End  and  Side  Views  Williams-Abbott  Generator, 
pole-pieces  are  bent  back  and  up  so  as  to  form  a  seat  for  the  per- 
manent magnets,  which  are  secured  in  place  by  two  bolts  passing, 
respectively,  between  the  two  outside  and  the  center  magnets. 
The  gear-wheels  in  this  instrument  are  cut  with  a  very  wide  face 
to  prevent  wear. 


io8  AMERICAN    TELEPHONE   PRACTICE. 

The  ringer  shown  in  Fig.  101  is  unique  and  embodies  several 
desirable  features:  The  two  ends  of  the  U-shaped  permanent 
magnet  are  of  the  same  polarity — say,  south — and  its  middle  is 
of  the  opposite  polarity.  (Hence  the  name  tripolar,  frequently 
applied  to  this  ringer.)  The  two  coils  are  mounted  on  the  iron 
cross  bar,  extending  between  the  legs  of  the  permanent  magnet. 

The  poles  of  these  electromagnets  therefore  partake  of  the 
polarity  of  the  permanent  magnet  ends, — that  is,  south, — while 
the  armature,  supported  from  the  center  of  the  permanent  magnet, 
becomes  of  north  polarity.  The  action  of  this  ringer  is  usually 
misunderstood  at  first  sight,  the  natural  supposition  being  that 
the  limbs  of  the  permanent  magnet  are  of  opposite  polarity. 
Besides  being  a  very  efficient  ringer,  this  has  the  advantage  of 
having  its  working  parts  inclosed  by  the  rigid  permanent 
magnet,  which  serves  to  protect  them  from  mechanical  injury. 

In  Figs.    102,    103,    104,  105,   and    106  are   shown   the   details 


Fig.  101. — Williams-Abbott  Ringer. 

of  a  unique  magneto-generator  and  ringer  recently  put  on  the 
market  by  the  Williams  Electric  Company  of  Cleveland.  This 
apparatus  is  the  design  of  Mr.  J.  A.  Williams,  and  embodies  such 
radical  departures  from  the  details  of  the  ordinary  magneto  as  to 
warrant  a  somewhat  minute  description. 

The  magnets  of  the  generator  are  formed  of  heavy  bars  of 
steel,  |  of.  an  inch  thick  by  ij  inch  wide.  They  are  placed 
close  together,  thus  covering  the  entire  length  of  the  pole-pieces 
and  securing  a  maximum  magnetic  density  across  the  pole  faces. 

The  pole-pieces  are  punched  from  sheet  iron,  and  are  ac- 
curately formed  to  inclose  the  space  in  which  the  armature  turns. 

The  end  plates,  which  hold  the  crank-shaft  and  armature  bear- 
ings, are  punched  from  heavy  sheet  brass,  and  have  riveted  into 
them  long  brass  bearings,  which  is  an  important  feature  in  secur- 
ing good  wearing  qualities. 


COMMERCIAL    CALLING  APPARATUS. 


109 


The  end  plates  and  the  pole-pieces  are  secured  by  screws,  as 
shown  in  Fig.  104,  to  horizontal  brass  plates  above  and  below  the 
armature.  The  method  of  affording  a  path  for  the  magnetic 
lines  from  the  permanent  magnets  to  the  pole-pieces  is  ingenious. 
Two  sheet-iron  contact  plates  are,  provided,  one  for  each  pole- 
piece.  Each  of  these  has  two  ears  turned  inwardly  to  hold  it 
in  position  on  the  pole-piece,  and  also  four  ears  turned  out- 
wardly to  hold  the  permanent  magnets  in  their  proper  place  on 
the  generator. 

Three  portions  of  each  contact  plate  are  left  flat  or  straight,  so 
as  to  make  a  good  contact  surface  on  the  permanent  magnets, 
and  two  portions  are  formed  so  as  to  make  curves  coincident 


Figs.  102  and  103. — Williams  Generator  and   Magneto-Bell. 

with  the  outside  curvature  of  the  pole-pieces.  This  arrangement 
is  for  the  purpose  of  securing  good  contact  between  the  perma- 
nent magnets  and  pole-pieces,  and  also  for  taking  the  magnetism 
from  the  permanent  magnets,  at  points  about  opposite  the  center 
of  the  armature,  thus  securing  a  somewhat  longer  effective 
magnet. 

The  gear  and  pinion  are  on  the  left-hand  side  of  the  machine 
instead  of  the  right,  as  is  usual,  the  object  being  to  get  them  as 
far  away  as  possible  from  the  uneven  strain  on  the  shaft  due  to 


HO  AMERICAN   TELEPHONE   PRACTICE. 

the  turning  of  the  crank.  The  feature  of  the  gear,  however,  is 
that  it  is  radially  corrugated,  as  shown  in  Figs.  103  and  104,  the 
object  of  this  being  to  secure  a  uniform  rate  of  wear  between 


Fig.  104. — Details  of  Williams  Generator. 

the  teeth  on  the  pinion  and  on  the  gear.  The  corrugations  on 
the  gear  cause  its  teeth  to  play  over  a  width  of  pinion  about  five 
times  as  wide  as  the  face  of  the  gear.  This,  in  view  of  the  fact 
that  the  gear  has  about  five  times  as  great  a  circumference  as  the 
pinion,  renders  the  wearing  surface  on  the  two  about  equal.  In 
order  to  prevent  the  large  gear  plowing  a  rut  in  the  small  one  by 
always  traveling  in  the  same  path,  the  ratio  of  the  teeth  is 
made  uneven,  there  being  134  teeth  on  the  large  gear  and  27  on 


N 

Figs.  105  and  106. — Williams  Ringer. 

the  pinion  ;  thus  the  armature  must  make  134  revolutions  before  a 
tooth,  on  the  gear  engages  the  same  tooth  on  the  pinion  twice. 

The  pinion  is  attached  to  the  armature  shaft  by  means  of  a  key 
and   machine   screw,  and   can  be  easily  taken   off  and   replaced 


COMMERCIAL    CALLING  APPARATUS.  117 

without  driving,  a  feature  of  great  convenience  in  making 
repairs. 

The  magnets  are  clamped  to  the  frame  of  the  machine  by  two 
brass  bolts,  these  being  long  enough  to  pass  through  the  bottom 
of  the  generator  box,  so  as  to  receive  nuts  for  securing  the 
generator  in  the  box.  The  automatic  shunt  of  this  machine  has 
already  been  described  in  Chapter  VIII. 

Not  less  unique  than  the  generator  of  this  machine  is  the 
ringer.  It  has  but  a  single  core,  which  is  parallel  with  the  vibrat- 
ing armature.  The  core  heads  or  end  pieces  are  formed  up  of 
Swedish  iron,  and  are  swaged  onto  the  core,  forming  a  very  per- 
fect magnetic  joint. 

The  permanent  magnet  of  the  ringer  is  U-shaped  and  of  the 
consequent-pole  type,  the  central  portion  being  of  one  polarity 
and  the  extremities  of  the  other.  The  armature  is  suspended 
from  the  central  and  consequent  pole. 

The  magnetic  circuit  of  this  ringer  is  well  shown  in  Fig.  106 
by  the  iron  filings  which  cling  to  the  poles.  It  will  be  noticed 
that  the  core  of  the  coil  really  forms  no  part  of  the  magnetic 
circuit  of  the  permanent  magnet,  as  each  of  its  ends  is  magnet- 
ized to  an  equal  extent  and  with  the  same  polarity. 

The  action  of  this  ringer  is  as  follows :  The  two  pole-pieces, 
which  form  at  the  same  time  the  heads  of  the  spool,  are  mag- 
netized by  the  ends  of  permanent  magnet,  with  the  same 
polarity — we  will  say,  north.  The  armature,  attached  to  the 
center  or  south  pole,  will  therefore  have  the  opposite  polarity- 
south.  When  a  current  traverses  the  coil,  it  gives  one  end  of 
the  core  and  the  corresponding  head  a  north  polarity  and  the 
other  end  and  head  a  south.  This  strengthens  the  magnetism 
already  possessed  by  the  former  and  weakens  that  of  the  latter, 
thus  causing  the  armature  to  be  attracted  by  the  stronger  pole. 
The  next  instant  the  current  reverses,  and  the  opposite  end  of 
the  armature' is  attracted. 

The  design  of  this  ringer  is  certainly  good.  The  length  of 
magnetic  circuit  through  which  the  magnetism  set  up  by  the  cur- 
rents in  the  coil  acts  is  reduced  to  a  minimum.  The  ends  of 
the  core  are  subjected  to  a  normal  magnetic  stress,  yet  the  core 
forms  no  part  of  the  normal  magnetic  circuit.  It  is  therefore 
rendered  very  sensitive  to  changes  in  the  magnetizing  force  due 
to  the  coil,  because  the  normal  magnetic  flux  through  the  core  is 
nil.  • 

Fig.  102  shows  the  complete  instrument  provided  also  with  a 
long-lever  gravity  switch  hook,  with  Craig  silver  contact-pieces 


112  AMERICAN    TELEPHONE  PRACTICE. 

attached  to  a  hard-rubber  block,  which,  together  with  the  hook, 
is  mounted  in  the  bell  box  in  such  a  way  as  to  make  the  switch 
hook  self-contained. 

The  box  is  wired  throughout  with  tinned  copper  wire,  which 
rests  between  tinned  spring  washers,  secured  by  entering  the 
binding  posts,  thus  forming  spring  contacts,  all  other  contacts 
being  soldered. 

The  regular  io,OOO-ohm  magneto  set  usually  has  an  armature 
wound  with  No.  35  or  No.  36  B.  &  S.  silk-covered  wire,  to  resist- 
ances varying  from  400  to  550  ohms.  The  ringer  magnets  are 
usually  wound  with  No.  31  B.  &  S.  wire  and  have  a  resistance  of 
from  75  to  100  ohms.  The  ordinary  construction  of  ringer  mag- 
nets is  to  drive  the  fiber  heads  directly  onto  the  cores,  and  after 
insulating  the  surface  of  the  latter  to  wind  the  spools  thus  formed 
with  silk-insulated  wire  of  the  desired  size.  Fig.  107  shows  one  of 


Fig.  107. — Varley  Ringer  Coil. 

the  sectional  coils  of  the  Varley  Duplex  Magnet  Company.  Their 
coils  are  wound  separately,  with  bare  wire  separated  by  silk  thread. 
After  winding,  the  coils  may  be  slipped  on  the  core  and  locked  by 
the  end  washer  or  head.  The  advantages  of  this  construction  are 
in  the  ease  of  replacing  burned-out  or  otherwise  injured  coils,  and 
also  in  the  perfect  uniformity  of  the  winding.  Bare-wire  winding 
will  probably  play  an  important  part  in  telephony  of  the  future. 

Where  many  instruments  are  to  be  placed  in  series  on  a  party 
line  the  ringer  magnets  should  be  made  as  low  as  40  ohms,  in 
order  to  reduce  the  amount  of  self-induction  through  which  it  is 
necessary  to  talk.  This  is  a  practice  little  followed  ;  but  good, 
nevertheless,  when  instruments  must  be  placed  in  series. 

For  bridging  work  the  conditions  are  changed.  The  generator 
must  have  greater  current  capacity,  and  for  that  reason  a  larger 
wire  in  the  winding  is  necessary.  This  necessitates  fewer  turns 
for  the  same  winding  space  and  a  consequent  loss  in  voltage. 
In  some  long  lines  the  voltage  must  necessarily  be  maintained, 
and  in  order  to  make  up  for  the  loss  in  this  respect,  due  to  fewer 


COMMERCIAL    CALLING  APPARATUS.  113 

turns,  the  field  magnets  may  be  made  stronger  and  the  armature 
longer.  A  good  generator  for  bridging  work  may  be  wound 
with  No.  33  wire  to  a  resistance  of  350  ohms. 

The  ringer  magnets  for  bridging  work  must  possess  a  very  high 
degree  of  self-induction.  This  should  be  obtained  by  winding 
them  to  a  high  resistance  with  a  comparatively  coarse  wire,  so  as 
to  obtain  a  large  number  of  turns  in  the  winding.  The  length 
of  the  cores  is  increased  for  the  double  purpose  of  getting  more 
iron  in  the  magnetic  circuit,  and  therefore  a  higher  retardation, 
and  also  for  affording  a  greater  amount  of  room  for  the  winding. 
The  Western  Electric  Comp;uiy  wind  their  coils  to  a  resistance 
of  lOOOohms,  using  No.  33  single-silk  magnet  wire.  Many  other 
companies  use  No.  38  wire  and  wind  to  a  resistance  of  1200  or 
1600  ohms.  This  does  not  give  such  good  results,  however,  as 
using  the  coarser  wire  and  the  lower  resistance  and  long  cores. 
Some  companies  wind,  or  once  wound,  their  bridging-bell  mag- 
nets partly  with  German-silver  wire,  in  order  to  make  a  high 
resistance  at  a  low  cost.  They  should  learn,  however,  that  resist- 
ance in  itself  is  not  the  thing  desired,  but  a  great  number  of  turns 
in  the  winding,  which,  of  course,  incidentally  produces  a  high 
resistance. 

CONSTANTLY   DRIVEN   GENERATORS. 

In  telephone  exchanges,  constantly  driven  generators  are  em- 
ployed for  ringing  purposes,  in  order  that  the  operators  may  be 


Fig.  108. — Western  Power  Generator. 

saved  the  labor  of  manually  turning  a  crank  every  time  a  subscri- 
ber is  called  up. 


114  AMERICAN    TELEPHONE  PRACTICE. 

Fig.  108  shows  one  type  of  machine  for  this   purpose.     It  is 
merely  a  magneto-generator  provided  with  very  long  and  powerful 


Fig.  109. — Holtzer-Cabot  Belt-Driven  Magneto. 

permanent  magnets.     The  machine  is  mounted   on  a   slate  base 
and  driven  by  means  of  a  grooved  pulley,  mounted  on  a  separate 


Fig.  1 10. — Motor-Generator. 

shaft  connected  to  the  armature  shaft  by  a  short  flexible   rubber 
coupling. 

Machines  of  this  type  are  suitable  for   small    exchanges,  and 
may   be    driven  by  any  available  source  of   power.      They    are 


COMMERCIAL    CALLING  APPARATUS.  115 

frequently  placed  in  an  electric  light  or  power  plant,  where  they 
may  be  constantly  driven.  Wires  are  then  run  from  them  to  the 
exchange,  from  which  the  current  is  sent  out  over  the  various 
subscribers'  lines  as  desired. 

Another   convenient  method  of    obtaining  power  for   switch- 
board generators  is  by  the  use  (of  small  water  motors,  run   from 


Fig.  ill. — Motor-Generator  for  Battery-Charging. 

city  water  mains.  These  are  specially  built  for  this  purpose, 
and  consume  but  little  water. 

A  more  convenient  source  of  power  is  an  electric  motor ;  and 
Fig.  109  shows  a  Holtzer-Cabot  magneto  belted  to  a  small  direct- 
current  motor,  also  manufactured  by  that  concern.  Convenient 
forms  of  alternating-current  motors  are  also  made  for  running 
generators  for  this  purpose. 

All  the  machines  so  far  mentioned  are  suitable  for  exchanges 
of  500  subscribers  and  under.  For  large  exchanges,  and  also 
many  smaller  ones,  the  motor-generator,  types  of  which  are 
shown  in  Figs,  no,  ui,and  112,  are  being  commonly  used. 
The  one  shown  in  Fig.  no  is  manufactured  by  the  Holtzer- 
Cabot  Co.,  and  is  provided  with  a  double  winding,  on  a  single 
armature  core,  the  same  field  serving  for  both  windings.  The 


n6 


AMERICAN   TELEPHONE   PRACTICE. 


side  having  the  commutator  is  the  motor  side,  and  may  be 
wound  for  any  desired  direct  voltage.  The  right-hand  side  is 
provided  with  collector  rings  for  supplying  alternating  current 
for  ringing  purposes  at  a  pressure  of  75  volts. 

Fig.  1 1 1  shows  a  machine  of  similar  type  manufactured  by 
the  Cre.cker- Wheeler  Co.  This  furnishes  alternating  ringing 
current,  and  besides  is  provided  with  an  extra  winding  for  sup- 
plying low-voltage  direct  current,  for  the  purpose  of  charging 
storage  batteries. 

In  Fig.  112  is  shown  a  similar  machine  for  charging  storage 
batteries  in  telephone  work,  and  provided  with  an  automatic 


Fi<j.  112. — Motor-Generator  with  Automatic  Circuit-Breaker. 


circuit-breaker  on  the  dynamo  side.  Considerable  trouble  has 
been  experienced  in  storage-battery  work,  due  to  leaving  the 
battery  in  circuit  with  the  generator  while  the  latter  was  not 
running.  This  allows  the  battery  to  discharge  itself  through  the 
armature  of  the  generator,  tending  to  cause  the  latter  to  turn 
as  a. motor.  In  order  to  prevent  this,  one  of  the  pole-pieces  of 
the  machine  is  hinged  at  the  base,  where  it  joins  the  bed  plate. 
This  pole-piece  is  normally  held  away  from  the  armature  by  a 


COMMERCIAL    CALLING  APPARATUS.  II? 

spring,  and  in  this  position  the  circuit-opening  switch,  shown  on 
top  of  the  field  magnets,  is  open.  When  current  is  supplied  to 
the  machine  its  magnetism  causes  the  pole-piece  to  draw  close 
to  the  armature,  thus  closing  the  battery  circuit.  As  soon  as 
the  machine  stops  the  magnetism  weakens,  and  the  circuit  is  again 
opened. 


CHAPTER  XI. 

THE   TELEPHONE    RELAY   OR   REPEATER. 

ONE  of  the  most  attractive  fields  of  research  and  invention  in 
telephony  has  been  that  of  the  telephone  relay  or  repeater.  It 
has  been  very  natural  to  suppose  that  the  principle  of  repeating 
now  used  so  successfully  and  extensively  on  long  telegraph  lines 
could  be  used  with  equal  advantage  on  long  telephone  lines. 
The  idea  is  very  simple,  and  involves  merely  the  placing  of  a 
microphone  contact  in  operative  relation  with  the  diaphragm  of 
a  receiver  connected  in  the  first  line  circuit,  and  causing  the 
changes  produced  in  the  resistance  of  this  contact,  when  acted 
upon  by  the  receiver  diaphragm,  to  vary  the  strength  of  a  cur- 
rent in  a  local  circuit,  which  circuit  would  in  turn  act  inductively 
on  the  second  line  wire  with  reinforced  energy. 

This  method  is  outlined  in  Fig.  113,  where  A  is  the  transmit- 
ting station,  being  provided  with  a  transmitter,  T,  battery,  B,  and 


Fig.  113.— Simple  Relay  Circuit. 

induction  coil,  I.  L  is  the  transmitting  line,  having  connected 
in  its  circuit  the  coil  of  a  receiver,  M.  D  is  the  vibrating 
diaphragm  of  this  receiver  against  the  center  of  which  rests  a 
pair  of  microphone  contacts,  which  may  be  the  same  as  those 
in  the  Blake  transmitter,  or  of  any  other  type.  This  micro- 
phone contact  must  be  so  arranged  with  respect  to  the  receiver 
diaphragm  that  any  vibrations  of  the  latter  will  be  imparted  to 
the  former,  thus  causing  them  to  vary  their  resistance  in  exactly 
the  same  manner  as  if  acted  upon  directly  by  sound  waves.  The 
microphone  contact,  C,  serves  to  vary  the  resistance  of  a  local 
circuit  containing  a  battery,  B ',  and  the  primary  of  an  induction 
coil,  /'.  L  is  the  receiving  line,  containing  the  secondary  of  the 
induction  coil,  /',  at  the  relay  station,  and  the  receiver,  R,  at 
the  receiving  station,  A'.  Any  changes  in  current  in  the  local 
circuit  at  the  station,  A,  produced  by  the  operator  at  the  trans- 

118 


THE    TELEPHONE  RELAY  OR   REPEATER.  119 

mitter,  T,  will  induce  alternating  currents  in  the  line,  L,  in  the 
ordinary  manner,  which  will  cause  the  diaphragm,  D,  to  vibrate  as 
in  everyday  practice.  The  vibrations  of  D  will  be  imparted  to 
the  microphone  contact,  C,  which  will  produce  changes  in  the 
current  flowing  in  the  local  circuit  at  the  relay  station  correspond- 
ing to  those  taking  place  in  the  local  circuit  at  station,  A.  These 
changes  will  act  inductively  on  the  line  circuit,  L ',  in  the  ordinary 
manner,  the  receiver,  R,  finally  reproducing  the  sound. 

Such  an  arrangement  as  this  will  do  its  work  well,  but  it  is 
quite  evident  that  the  transmission  may  be  effected  only  in  one 
direction.  When  it  is  desired  to  transmit  from  station  A  to  A, 
a  separate  circuit  would  ordinarily  have  to  be  used.  Much  diffi- 
culty was  experienced  in  making  a  two-way  repeater,  for  no  auto- 
matic switch  could  be  arranged  which  would  bring  about  the 
changes  of  circuit  required  when  the  transmitting  station  desired 
to  become  the  receiving.  Many  attempts  were  made  to  associ- 
ate two  relays  with  the  line  circuits  in  such  manner  that  no  in- 
terference would  occur.  The  difficulties  involved  in  this  were, 
however,  great ;  and  chief  among  them  was  the  fact  that  two 
relays  when  associated  with  the  same  pair  of  lines  would  almost 
invariably  set  up  a  singing  sound,  due  to  the  mutual  action  be- 
tween the  two  ;  for  instance,  a  slight  vibration  of  the  diaphragm 
of  one  relay  would  produce  changes  in  current  in  the  local  cir- 
cuit, which  would  act  upon  the  diaphragm  of  the  other  relay, 
producing  another  change  of  current,  which  would  in  turn  react 
upon  the  first  relay.  This  action  is  somewhat  analogous  to  that 
produced  by  holding  the  earpiece  of  a  telephone  receiver 
directly  in  front  of  the  mouthpiece  of  a  good  granular-carbon 
transmitter ;  the  singing  or  shrieking  noise  set  up  when  a 
proper  adjustment  is  obtained  in  this  case  being  due  to  the  fact 
that  the  sound  waves  set  up  by  the  receiver  diaphragm  act  upon 
the  transmitter  diaphragm,  which  in  turn  causes  currents  to  flow 
through  the  receiver  coil,  causing  its  diaphragm  to  vibrate  still 
more  strongly.  This  defect,  however,  was  finally  overcome, 
several  inventors  having  produced  two-way  relays  which  were 
successful  in  so  far  as  they  would  operate  in  either  direction 
with  equal  facility,  and  with  a  fair  degree  of  clearness. 

One  of  these  systems,  devised  by  Edison,  is  shown  in  Fig.  114, 
in  which  A  and  A  are  the  telephone  stations,  each  arranged 
in  the  ordinary  manner.  M  is  the  magnet  of  the  relay  receiver, 
the  coil  of  which  is  included  in  a  local  circuit  containing  the 
secondary,  3,  of  an  induction  coil.  The  primary  winding  of  this 
coil  is  divided  into  two  parts,  I  and  2,  these  parts  being  connected 


120  AMERICAN    TELEPHONE   PRACTICE. 

together  in  one  side  of  the  combined  circuit  of  the  two  lines,  L 
and  L.  Between  the  juncture  of  these  two  primary  coils  and 
the  opposite  side  of  the  line  is  connected  the  secondary  coil,  4, 
of  an  ordinary  induction  coil.  The  primary  coil,  5,  of  this  latter 
induction  coil  is  connected  in  a  local  circuit  containing  the  relay 
microphone  contact,  C,  and  the  local  battery,  B" .  Assuming 
that  station,  A,  is  for  the  time  being  the  transmitting  station, 
currents  set  up  in  the  line  circuit,  L,  will  divide  at  the  relay 
station,  part  passing  through  the  coils,  I  and  4,  and  back  to  the 
transmitting  station,  and  the  other  part  passing  through  the 
primary  coils,  I  and  2,  in  series  and  to  the  receiving  station 


Fig.  1 14.  —Two-way  Relay  Circuit. 

direct.  The  current  passing  through  the  coil,  4,  will,  however, 
under  ordinary  circumstances,  be  by  far  the  greater  on  account 
of  the  high  resistance  of  the  long  line,  L '.  The  current  passing 
through  the  coils,  I  and  2,  however,  will  act  inductively  upon 
the  coil,  3,  thus  causing  currents  to  flow  through  the  coil  on  mag- 
net, M,  and  produce  changes  in  the  contact  resistance  of  the  mi- 
crophone. These  changes  will  cause  fluctuations  in  the  current 
in  the  local  circuit,  which  fluctuations  will  act  through  the  pri- 
mary coil,  5,  upon  the  secondary  coil,  4,  and  cause  currents  of 
considerable  comparative  strength  to  flow  in  the  line  circuit,  L ', 
to  the  receiving  station,  A ' .  It  is  obvious  that  as  the  various 
circuits  at  the  relay  station  are  symmetrically  connected  with 
respect  to  the  two  lines,  L  and  Z/,  the  station,  A ',  may  in  turn 
serve  as  the  transmitting  station.  No  reactive  effect  between 
the  relay  transmitter  and  receiver  will  in  this  case  be  produced, 
and  the  means  for  preventing  this  forms  the  most  interesting 
portion  of  this  invention.  Whatever  currents  are  set  up  in  the 
coil,  4,  by  the  action  of  the  microphone  contact,  C,  will  divide 
equally  between  the  primary  coils,  I  and  2,  passing  through  them 
in  opposite  directions.  These  coils  will  therefore  act  differen- 
tially upon  the  coil,  3,  and  their  effects  will  be  neutralized.  No 
current  will  be  caused  to  flow  in  the  circuit  containing  coil,  3, 
and  the  relay  magnet,  M,  and  therefore  no  reactive  effect  will  be 
produced  upon  the  transmitter.  In  other  words,  any  current 


THE    TELEPHONE   RE  LA  Y   OR   REPEA  TER. 


121 


flowing  in  either  line  circuit  will  induce  currents  in  the  local 
circuit  containing  the  magnet,  M,  while  current <  set  up  in  the 
coil,  4,  by  virtue  of  currents  flowing  through  the  magnet,  M, 
will  produce  no  effect  in  turn  upon  the  coil,  3. 

A  great  many  improvements  have  been  made  in  the  mechani- 
cal construction  of  the  telephone  relay,  but  with  few  exceptions 
they  have  embodied  only  the  idea  of  combining  an  ordinary 
transmitter  with  an  ordinary  receiver.  In  1897,  however,  a  relay 
was  devised  by  Mr.  A.  W.  Erdman,  and  is  shown  in  Fig.  115, 


w 


Fig.  115. — Erdman  Telephone  Relay. 

and  embodies  probably  the  most  radical  departure  in  the  struc- 
ture of  telephone  repeaters  of  all  since  the  first  was  produced. 
In  this  figure,  L  is  the  transmitting  line  and  L?  the  receiving 
line.  H  is  the  diaphragm  of  the  receiving  instrument  and  is 
used  to  operate  the  balanced  valve,  F2,  which  by  its  motion  to 
and  fro  varies  the  flow  of  an  otherwise  constant  stream  of  air 
flowing  through  the  chamber,  C.  This  chamber  is  covered  by 
a  flexible  diaphragm,  D,  which  is  caused  to  vibrate  by  the 
changes  in  pressure  within  the  chamber  produced  by  the  motion 
of  the  valve,  F2,  The  diaphragm,  D,  serves  to  operate  a  micro- 
phone, T,  which  in  this  case  consists  of  the  variable  resistance 
button  of  the  solid-back  transmitter.  R  is  a  reservoir  containing 
compressed  air,  and  Fa  reducing  valve  by  which  the  amount  of 
air  escaping  through  the  chamber  may  be  regulated.  In  the 
balanced  valve,  F2,  E  is  a  flexible  diaphragm  and  A  a  movable 
portion  which  controls  the  outlet.  The  centers  of  the  diaphragm, 
JS,  and  of  the  valve  plate,  A,  are  connected  by  the  rod,  Ft  to  the 


122 


AMERICAN   TELEPHONE  PRACTICE. 


center  of  the  receiver  diaphragm,  H.  The  balancing  of  the 
valve,  F2,  renders  it  extremely  sensitive,  so  that  it  may  be  set  in 
motion  by  the  delicate  movements  of  the  diaphragm,  H.  In 
operation,  the  vibrations  of  the  diaphragm,  H,  caused  by  currents 
in  the  transmitting  line,  L,  cause  the  balanced  valve,  F2,  to  vary 
the  opening  of  the  air  outlet.  This  produces  changes  in  pressure 
within  the  air  chamber  under  the  diaphragm,  D,  which  cause 
that  diaphragm  to  vibrate  and  thus  actuate  the  microphone  in 
the  usual  way,  thus  causing  currents  to  flow  in  the  receiving  line, 
Z2,  in  the  usual  way.  No  reports  have  been  made  public  con- 
cerning the  results  obtained  in  actual  practice  with  this  repeater, 


Fig.  116. — Stone  Telephone  Relay. 

but  it  seems  that  it  may  be  a  step  toward  the  solution  of  this 
difficult  problem.  Instead  of  employing  the  mechanical  connec- 
tion commonly  used  between  the  diaphragms  of  the  transmitting 
and  receiving  mechanisms,  Mr.  Erdman  has,  in  his  current  of  air 
or  gas,  chosen  one  of  the  most  delicately  subtile  mediums  known. 
Another  relay,  devised  by  Mr.  John  S.  Stone  of  the  American 
Bell  Telephone  Co.,  is  shown  in  Fig.  116.  This  relay  differs  in 
the  essentials  of  its  construction  from  those  of  the  older  type 
only  in  that  its  entire  working  parts  are  inclosed  in  a  vacuum 
chamber.  The  repeater,  together  with  the  circuits  of  the  two 
connected  lines,  is  shown  in  this  figure,  in  which  T  is  the  trans- 
mitter of  the  sending  station  and  /  the  receiver  of  the  receiving 
station.  These  are  connected  with  the  repeater  bylines,  D  and  L, 
respectively,  the  line  circuits  being  associated  with  the  repeater  cir- 


THE    TELEPHONE  RELAY  OR   REPEATER.  123 

cuits  by  induction  coils,  /and  72,  in  the  usual  manner.  B  is  a  polar- 
ized electromagnet  whose  poles  are  in  proximity  to  the  diaphragm, 
D.  C  is  the  variable  resistance  button  of  a  solid-back  transmitter, 
the  front  electrode  of-which  is  rigidly  secured  to  the  center  of  the 
diaphragm,  Z),  while  the  back  electrode  is  rigidly  secured  by  means 
of  a  cross-piece  to  the  frame,  A, which  also  supports  the  diaphragm, 
D,  and  the  electromagnet,  B.  A  is  a  bell  jar  closely  fitted  to  the 
base,  by  by  an  air-tight  joint.  The  air  from  within  the  chamber 
may  be  withdrawn  by  the  pipe,  /^attached  to  an  air  pump.  It  is 
said  that  the  removal  of  the  air  from  within  the  chamber  brings 
about  a  decided  improvement  in  the  operation  of  the  repeater. 
Concerning  the  results  obtained,  Mr.  Stone  says,  "The  messages 
automatically  transferred  by  it  from  one  circuit  to  another  are 
reproduced  in  the  receiving  telephone  of  the  second  circuit  with 
a  well-defined  gain  in  volume  or  loudness,  and  without  any  sub- 
stantial distortion  or  offsetting  loss  in  clearness  of  articulation." 
If  this  claim  is  borne  out  in  practice,  the  production  of  this  relay 
should  prove  a  step  of  some  importance  in  the  matter  of  long- 
distance telephony. 

It  has  seemed  plausible  that  very  feeble  currents  received  at 
the  relay  station  would,  by  virtue  of  the  delicate  action  of  the 
microphone,  be  able  to  produce  comparatively  large  changes  in 
resistance  of  the  local  relay  circuit  associated  therewith,  and  that 
these  changes  in  resistance  would  produce  correspondingly  great 
changes  in  the  current  of  the  local  battery  at  that  station,  which 
changes  would  act  inductively  on  the  second  line  wire  with  per- 
haps as  much  energy  as  that  imparted  to  the  original  circuit.  As 
a  matter  of  fact,  however,  no  gain  in  the  volume  of  transmission 
has  ever  been  commercially  effected  by  this  method.  The  tele- 
phone repeater  may  be  made  to  work  perfectly  on  ordinary  lines, 
but  it  has  not  shown  its  ability  to  transmit  speech  between  two 
distant  points  any  better  than,  or  quite  as  well  as  could  be  done 
by  direct  transmission  without  the  use  of  the  relay  at  all.  The 
amount  of  energy  received  by  the  electromagnet  of  the  relay 
is  so  exceedingly  small  that  it  cannot  be  made  to  produce  the 
desired  mechanical  effect  upon  the  microphone  contact. 


CHAPTER  XII. 

SELF-INDUCTION   AND   CAPACITY. 

SELF-INDUCTION  and  capacity  play  such  important  parts  in  the 
question  of  long-distance  telephone  transmission,  and  seem  so 
little  understood  among  the  rank  and  file  of  telephone  workers 
and  users,  that  this  chapter  will  be  devoted  to  an  elementary  and 
non-mathematical  discussion  of  these  two  phenomena,  with  a 
view  to  explaining  their  existence  and  effect  in  a  simple  manner, 
rather  than  to  throw  any  new  light  upon  the  subject. 

Ohm's  law  states  that  for  a  steady  flow  of  electricity  in  a  given 
circuit  the  amount  of  current  in  amperes  is  equal  to  the  electro- 
motive force  expressed  in  volts,  divided  by  the  resistance  of  the 
circuit  expressed  in  ohms.  In  algebraic  form  this  becomes  the 
well-known  equation  : 


where  /  represents  the  current  in  amperes,  E  the  electromo- 
tive force  in  volts,  and  R  the  resistance  of  the  circuit  in  ohms. 
Knowing  any  two  of  the  above  quantities,  the  third  may  be 
determined  from  the  equation  already  given,  or  from  the  follow- 
ing, which  are  derived  from  it  : 

E  =  /ff, 

and  R  =  —  - 

These  three  equations,  which  are  merely  different  ways  of 
expressing  Ohm's  law,  are  the  most  useful  in  the  entire  science 
of  electricity.  It  is  unfortunate  for  an  easy  understanding  of 
telephony  that  these  equations  in  their  simple  forms  hold  true 
for  a  steady  flow  only,  .and  that  when  currents  which  are  rapidly 
changing  in  value  or  in  direction  are  considered,  we  must  face  a 
more  complex  set  of  conditions. 

An  electric  current  flowing  through  a  conductor  sets  up  a  field 
of  force  about  the  conductor  throughout  its  entire  length.  This 
field  of  force  consists  of  magnetic  lines  extending  in  closed 
curves  about  the  conductor,  and  is  often  termed  a  magnetic 


SELF-INDUCTION  AND   CAPACITY.  125 

whirl.  A  freely  suspended  magnetic  needle  placed  within  this 
field  of  force  will  tend  to  assume  a  direction  corresponding  to 
the  direction  of  the  lines  of  force,  and  therefore  at  right  angles 
to  the  conductor. 

If  the  current  flowing  in  the  conductor  is  maintained  at  a  con- 
stant value  and  in  the  same  direction,  the  field  of  force  about  the 
conductor  will  not  change.  On  the  other  hand,  if  the  current 
strength  fluctuates,  the  field  of  force  will  become  more  intense 
and  will  expand  while  the  current  strength  is  increasing,  and  will 
become  less  intense  and  will  therefore  contract  while  the  current 
strength  is  decreasing.  If  the  current  changes  its  direction,  the 
field  of  force  existing  is  entirely  destroyed,  and  is  built  up  in  an 
opposite  direction  at  every  such  change. 

Whenever  there  is  such  a  relative  movement  between  a  con- 
ductor and  the  lines  of  force  of  a  magnetic  field  as  to  cause  the 
conductor  to  cut  the  lines,  or  the  lines  to  cut  the  conductor,  an 
electromotive  force  is  set  up  in  the  conductor  which  tends  to 
cause  a  current  to  flow.  The  direction  of  the  electromotive 
force  will  depend  on  the  direction  of  the  lines  and  on  the  move- 
ment of  the  conductor,  and  its  value  will  depend  on  the  rate  of 
cutting.  The  field  of  force  may  be  set  up  either  by  a  magnet  or 
by  a  conductor  carrying  a  current,  and  in  either  case  the  phe- 
nomenon just  described  is  called  electromagnetic  induction. 

If  two  wires  are  formed  into  parallel  coils,  each  having  a  large 
number  of  turns,  then  the  lines  of  the  field  of  force  set  up  by 
the  coil  carrying  the  current  will  cut  many  of  the  turns  of  the 
other  wire,  thus  inducing  an  electromotive  force  in  each  turn  ; 
the  result  being  that  the  sum  of  all  the  electromotive  forces  so 
induced  will  be  added,  thus  producing  a  much  greater  effect  than 
if  each  wire  consisted  of  but  a  single  turn.  Furthermore,  if  the 
two  coils  are  wrapped  about  an  iron  core,  the  field  of  force  due 
to  the  coil  carrying  the  current  will  be  greatly  strengthened,  and 
therefore  the  electromotive  force  induced  in  the  second  coil  will 
be  greatly  increased,  owing  to  the  greater  rate  of  cutting  of  lines 
caused  by  the  changes  in  the  strength  of  the  current.  This  is 
due  to  the  fact  that  a  given  magnetizing  force,  or  force  which 
tends  to  set  up  a  field  of  force,  will  produce  a  greater  number  of 
lines  in  iron  than  in  air. 

It  is  evident  that  in  a  coil  of  wire  carrying  a  current  each  turn 
of  the  coil  is  surrounded  by  a  field  of  force,  and  that  each  turn 
must  therefore  lie  more  or  less  within  the  fields  of  force  of  all 
the  other  turns.  Each  turn  will  therefore  have  an  inductive 
action  upon  all  the  other  turns  when  the  current  through  the  coil 


126  AMERICAN    TELEPHONE   PRACTICE. 

is  varying.  Whenever  a  diminution  of  the  current  occurs  the 
decreasing  number  of  lines  of  force  set  up  by  any  one  turn  will 
act  on  each  of  the  other  turns  to  induce  an  electromotive  force 
tending  to  cause  a  current  to  flow  in  the  same  direction.  The 
decreasing  field  of  force  around  each  one  of  the  turns  will  act  in 
a  like  manner  on  all  of  the  other  turns,  and  as  all  of  the  electro- 
motive forces  in  all  of  the  turns  will  be  in  the  same  direction  as 
the  current  which  is  already  flowing  in  the  coil,  their  effects  will 
be  added  and  will  tend  to  prolong  the  flow  of  current.  On  the 
other  hand,  an  increase  in  the  current  will  cause  an  increasing 
number  of  lines  to  surround  each  turn,  and  this  increase  around 
any  one  turn  will  induce  electromotive  forces  in  each  of  the 
other  turns  in  the  opposite  direction  to  that  producing  the  cur- 
rent already  flowing.  This  phenomenon  of  induction  between 
the  various  parts  of  a  single  coil  of  wire  each  on  the  other  is 
termed  self-induction. 

In  view  of  the  fact  that  a  decreasing  current  induces  an  elec- 
tromotive force  tending  to  produce  a  current  in  the  same  direc- 
tion as  that  already  flowing,  while  an  increasing  current  induces 
an  electromotive  force  tending  to  produce  current  in  the  oppo- 
site direction,  it  follows  that  the  general  effect  of  self-induction 
in  a  circuit  is  to  tend  to  prevent  any  changes  in  current  from 
taking  place  in  that  circuit.  This  accounts  for  the  fact  that  coils 
of  wire,  such  as  those  forming  electromagnets,  tend  to  so  greatly 
reduce  the  flow  of  voice  currents  through  them  ;  one  of  the  best 
illustrations  being  that  used  in  the  bridging  bell  system  of  party 
lines,  where  the  ringer  magnets  are  purposely  wound  with  a  great 
number  of  turns  and  provided  with  long,  heavy  iron  cores  for 
the  purpose  of  increasing  the  self-induction. 

It  is  quite  evident  that  in  circuits  containing  self-induction  and 
subject  to  rapidly  fluctuating  electromotive  forces,  the  tendency 
of  self-induction  to  prevent  changes  in  the  current  will  always 
cause  any  change  in  current  to  lag  slightly  behind  the  electro- 
motive force  which  produces  it.  Where  the  electromotive  force 
impressed  upon  a  circuit  varies  according  to  the  law  of  sines,  the 
electromotive  force  produced  by  self-induction  lags  a  quarter  of  a 
phase  or  90°  behind  the  current  flowing  in  the  circuit.  That 
this  is  so  may  be  seen  from  the  fact  that  the  electromotive  force 
of  self-induction  is  a  maximum  when  the  current  producing  it  is 
changing  most  rapidly,  and  is  zero  when  the  current  producing  it 
is  not  changing  at  all.  The  maximum  rate  of  change  of  the 
current  flowing  in  a  circuit  occurs  when  the  current  is  passing 
through  zero,  and  its  minimum  rate  of  change  occurs  at  the 


SELF-INDUCTION  AND   CAPACH^Y.  127 

crests  of  the  wave,  that  is,  at  its  maximum  points.  It  therefore 
follows  that  the  electromotive  force  of  self-induction  is  a  maxi- 
mum when  the  current  in  the  circuit  is  zero,  and  is  zero  when 
the  current  is  a  maximum.  This  evidently  indicates  a  phase  dif- 
ference of  90°,  and  we  have  already  seen  that  this  phase  differ- 
ence is  a  lagging  rather  than  a  leading  one. 

In  circuits  containing  only  noii-inductive  resistance  the  electro- 
motive force  impressed  upon  the  circuit  has  only  to  overcome 
the  ohrnic  resistance,  and  the  value  of  the  current  may  be 
obtained  at  any  time  by  a  direct  application  of  Ohm's  law. 
Where  self-induction,  however,  is  added,  the  impressed  electro- 
motive force,  if  it  be  a  varying  one,  must  overcome  not  only  the 
ohmic  resistance,  but  the  electromotive  force  of  self-induction  ; 
and  then  the  current  equation  becomes 

Current  =     E^ectromot^ve  Force 
Impedance 

The  word  impedance  in  this  equation  may  be  termed  the 
apparent  resistance,  and  the  apparent  resistance  in  circuits  hav- 
ing self-induction  is  always  greater  than  the  ohmic  resistance. 
In  fact,  Z,  the  impedance,  is  equal  to 


where  f  is  the  frequency  of  alternations  and  L  is  the  coeffi- 
cient of  self-induction — a  term  denoting  the  total  number  of  lines 
of  force  set  up  in  a  given  coil  when  traversed  by  current  of  unit 
strength.  The  equation  ot  the  flow  of  current,  /,  may  then  be 
written 

/=  E 


which  is  the  equivalent  of  saying  that  the  current  flowing  is 
equal  to  the  electromotive  force  divided  by  the  apparent  resist- 
ance. 

The  current  flowing  in  a  circuit  in  which  self-induction  and 
resistance  are  present  is  the  resultant  of  that  produced  by  the 
impressed  electromotive  force  and  the  electromotive  force  of 
self-induction.  The  greater  the  electromotive  force  of  self-induc- 
tion the  greater  will  be  the  lag  of  the  current  behind  the 
impressed  electromotive  force.  Furthermore,  the  greater  the 
self-induction  the  greater  will  be  the  apparent  resistance 
or  impedance,  and  consequently  the  smaller  will  be  the 


128  AMERICAN    TELEPHONE   PRACTICE. 

current  flowing.  The  above  formula  applies  only  to  currents 
varying  according  to  the  sine  law  ;  but  telephone  currents  do  not 
vary  according  to  this  law,  or  according  to  any  other  definite 
law,  so  far  as  we  have  been  able  to  determine.  This  does  not, 
however,  destroy  the  significance  of  the  formula  as  applied  to 
telephony.  Fourier's  theorem  states  that  any  complex  periodic 
wave  motion  may  be  considered  as  being  made  up  of  a  number 
of  simple  wave  motions  having  I,  2,  3,  4,  etc.,  times  the  rate  of 
vibration  of  the  complex  wave  motion.  Telephone  currents  are 
very  complex,  and  are  composed  not  only  of  a  fundamental  tone, 
but  of  many  overtones  ;  it  is  by  the  various  blending  of  these 
overtones,  with  regard  to  their  relative  loudness  and  their  relative 
position  in  phase  with  respect  to  each  other,  that  articulate 
speech  is  produced.  A  consideration  of  the  formula  for  the  flow 
of  current,  just  given,  shows  that  the  effect  of  self-induction  is 
greater  upon  currents  of  high  frequency  than  upon  those  of  low 
frequency,  for  as  f,  the  frequency,  increases,  the  value  of  the 
impedance  or  the  apparent  resistance  increases,  and,  therefore, 
the  value  of  the  current  decreases.  In  other  words,  self-induc- 
tion tends  to  weed  out  the  higher  overtones  in  preference  to  the 
lower  ones  and  the  fundamental  tone,  thus  rendering  speech 
indistinct,  as  well  as  reducing  its  volume. 

Every  insulated  conductor  is  capable  of  receiving  a  certain 
charge  when  subjected  to  an  electromotive  force;  for  instance,  if 
a  metallic  plate  insulated  from  all  surrounding  bodies  is  connected 
to  one  terminal  of  a  battery  the  other  terminal  of  which  is 
grounded,  a  certain  amount  of  electricity  will  flow  into  the  plate 
until  its  potential  is  raised  to  that  of  the  battery  terminal.  The 
plate  is  then  said  to  be  charged,  and  the  amount  of  electricity 
held  by  it  determines  its  capacity.  The  charge  of  electricity  on 
the  plate  will  be  considered  positive  or  negative,  according  to 
whether  the  positive  or  negative  terminal  of  the  battery,  or  other 
charging  source,  was  connected  with  it. 

It  is  well  known  that  no  charge  exists  by  itself — there  is  always 
an  equal  and  opposite  charge  induced  by  it  upon  neighboring 
bodies.  It  is  also  well  known  that  like  charges  repel  each  other, 
while  unlike  charges  attract;  that  if  an  uncharged  body  be 
brought  near  a  charged  body  an  equal  and  opposite  charge  will 
be  induced  on  the  side  of  the  uncharged  body  which  is  toward 
the  charged  body,  and  that  similarly  a  charge  on  the  same  sign 
as  that  on  the  charged  body  will  be  induced  on  the  opposite  side 
of  the  uncharged  body.  If  now  the  body  which  was  originally 
uncharged  is  connected  with  the  ground,  this  latter  charge — that 


SELF-INDUCTION  AND   CAPACITY.  129 

is,  the  one  of  the  same  sign  as  the  original  charge — will  be  driven 
to  the  ground,  while  the  charge  of  opposite  sign  will  still  be 
attracted  by  the  charge  on  the  first  body.  The  second  body  will 
therefore  be  charged,  although  it  has  not  been  in  contact  with 
the  first.  The  action  between  charges  of  electricity  taking  place 
through  an  insulating  medium  is  called  electrostatic  induction. 
It  is  found  that  where  two  conductors  are  placed  side  by  side,  but 
insulated  from  each  other,  the  capacity  of  each  will  be  greater 
than  if  the  other  were  not  present.  For  the  purpose  of  holding 
charges  in  this  manner  the  well-known  Leyden  jars  have  long 
been  in  use.  They  are  usually  made  by  coating  a  glass  jar  inside 
and  out  with  a  layer  of  tin-foil  to  within  a  few  inches  of  the  top. 
The  outer  coating  is  usually  connected  with  the  ground,  while  the 
inner  coating  is  connected  with  a  metallic  rod  approaching  it 
through  the  mouth  of  the  jar.  If  the  inner  coating  is  connected 
with  a  source  of  electromotive  force,  a  current  lasting  but  an 
instant  will  flow  into  the  coating,  producing  a  charge.  This 
charge,  which  we  will  say  is  positive,  will  attract  a  nearly  equal 
negative  charge  to  the  outer  coating,  repelling  an  equal  positive 
charge  to  the  earth,  as  already  described.  The  amount  of  charge 
which  the  inner  coating  will  receive  under  these  circumstances  is 
very  much  greater  than  if  the  outer  coating  were  not  present,  and 
the  capacity  of  the  inner  coating  is  therefore  much  higher  than 
before.  A  device  such  as  the  Leyden  jar  is  called  a  condenser. 

The  capacity  of  a  condenser  is  increased  as  the  area  of  the 
conducting  surface  is  increased;  is  increased  as  the  distance 
between  the  conductors  is  diminished,  and  may  be  increased  or 
diminished  by  using  different  kinds  of  insulating  material  between 
the  conductors.  The  medium  separating  the  conductors  is  called 
the  dielectric,  and,  as  stated  above,  upon  it  depends  to  a  great 
extent  the  efficiency  of  a  condenser.  Several  condensers  built 
exactly  alike,  so  far  as  size  of  plates  and  the  distance  between 
them  are  concerned,  and  using  different  materials  for  dielectrics, 
will  be  found  to  have  different  capacities.  This  difference  is  due 
to  a  peculiar  property  possessed  to  different  degrees  by  different 
dielectrics  and  called  specific  inductive  capacity. 

The  specific  inductive  capacity  of  a  dielectric  is  a  measure  of 
that  quality  which  enables  the  dielectric  to  hold  a  charge  between 
two  conductors,  as  in  a  condenser.  The  specific  inductive  ca- 
pacity of  air  is  taken  as  a  standard  and  is  for  convenience  con- 
sidered as  unity  ;  it  is  lower  than  that  of  any  other  known 
substance  excepting,  perhaps,  hydrogen.  If  two  condensers 
having  plates  of  equal  size  and  distance  apart  are  constructed 


130  AMERICAN    TELEPHONE   PRACTICE. 

with  dielectrics  respectively  of  air  and  guttapercha,  it  will  be 
found  that  the  condenser  having  the  dielectric  of  guttapercha 
will  receive  a  charge  nearly  2\  times  as  great  as  the  condenser 
having  the  dielectric  of  air.  The  actual  ratio  between  the  two 
is  2.462,  and  for  this  reason  the  specific  inductive  capacity  of 
guttapercha  is  said  to  be  2.462.  The  following  table  gives  the 
specific  inductive  capacities  of  some  of  the  more  important 
insulators : 

Air, I. oo 

Glass 3.013 

Shellac, 2.74 

Sulphur, 2.580 

Guttapercha,  ..........  2.462 

Ebonite, 2.284 

India-rubber,  ..........  2.220 

Paraffin,          ...........  1.994 

Carbonic  Acid 1.00036 

Hydrogen,     ...........  0.99967 

Vacuum,         ...........  0.99941 

It  is  probable  that  with  very  rapidly  varying  electromotive 
forces,  such  as  are  dealt  with  in  telephony,  the  specific  inductive 
capacities  of  the  various  substances  would  be  higher  in  com- 
parison with  air  than  those  indicated  by  this  table. 

Specific  inductive  capacity  is  a  very  important  consideration  in 
the  construction  of  cables  for  telephone  purposes.  In  the  con- 
struction of  these  cables  it  is  desirable,  as  will  be  shown  later,  to 
reduce  the  capacity  of  the  wires  of  the  cable  to  as  great  an 
extent  as  possible,  and  in  order  to  do  this  the  dielectric  is,  in  the 
best  forms  of  cables,  made  to  as  great  an  extent  as  possible  of 
dry  air.  On  the  other  hand,  in  the  construction  of  condensers 
it  is  desired  that  the  capacity  may  be  as  great  as  possible  for 
a  given  area  of  plates,  and  therefore  some  material  other  than 
air  is  used.  Paper  saturated  with  paraffin  is  perhaps  the  most 
commonly  used,  paraffin  having  about  twice  as  great  a  specific  in- 
ductive capacity  as  air,  and  moreover  lending  itself  readily  to  the 
purposes  of  insulation.  To  sum  up,  the  capacity  of  a  condenser 
varies  in  direct  proportion  as  the  area  of  its  plates,  inversely  as 
the  square  of  the  distance  between  the  plates,  and  directly  as  the 
specific  inductive  capacity  of  the  dielectric. 

The  effect  of  a  condenser  bridged  across  a  circuit  carrying  an 
alternating  current  is  to  absorb  a  portion  of  the  current  as  the 
electromotive  force  at  its  terminals  increases,  and  as  the  electro- 
motive force  decreases,  to  give  this  current  back  to  the  line. 
Consider  such  a  circuit  when  the  electromotive  force  active  in 
driving  current  through  it  begins  to  rise.  The  electromotive  force 
at  the  condenser  terminals  will  also  rise,  and  current  will  there- 


SELF-INDUCTION  AND   CAPACITY.  131 

fore  flow  into  the  condenser.  The  strength  of  this  current  will 
depend  directly  upon  the  rate  at  which  the  potential  at  the 
terminals  oT  the  condenser  is  changing.  When  the  electromotive 
force  acting  in  the  circuit  reaches  a  maximum,  the  potential  at 
the  condenser  terminals  will  also  be  a  maximum  and  will  for  an 
instant  cease  to  change.  At  this  point  the  condenser  is  fully 
charged,  but  as  the  electromotive  force  of  the  line  is  not  chang- 
ing no  more  current  flows  into  the  condenser ;  in  other  words, 
the  condenser  current  is  zero.  As  the  electromotive  force  in  the 
line  decreases,  current  will  flow  out  of  the  condenser  and  into  the 
line,  because  the  condenser  is  not  capable  of  holding  so  much 
charge  at  the  lower  potential.  The  flow  of  current  out  of  the 
condenser  reaches  a  maximum  when  the  electromotive  force  in 
the  line  is  changing  most  rapidly,  and  this  occurs  when  it  is 
passing  through  zero.  From  this  it  will  be  seen  that  the  con- 
denser current  is  zero  when  the  electromotive  force  in  the  line  is 
a  maximum,  and  is  a  maximum  when  the  electromotive  force  in 
the  line  is  zero.  This  indicates,  as  in  the  case  of  self-induction, 
a  phase  difference  of  90°,  or  a  quarter  of  a  cycle.  It  is  not  so 
easy  to  say  whether  this  phase  difference  is  lagging  or  leading, 
but  a  consideration  of  the  direction  of  flow  of  current  throughout 
the  cycle  will  throw  some  light  upon  the  subject.  , 

At  the  instant  when  the  current  flowing  in  the  line  (which  is 
in  exact  phase  with  the  active  electromotive  force  in  the  line  *) 
is  positive  and  at  a  maximum,  the  condenser  current  will  be 
zero.  As  the  active  electromotive  force  decreases  toward  zero 
the  condenser  current  increases,  but  in  a  different  direction,— 
negative, — because  current  is  now  flowing  out  of  the  condenser 
back  to  the  line.  As  the  active  electromotive  force  reaches  zero 
the  condenser  current  is  at  its  maximum  negative  value,  and  as 
the  active  electromotive  force  reaches  its  maximum  negative 
value  the  condenser  current  reaches  zero.  During  the  next  half- 
cycle  the  condenser  current  increases  to  a  positive  maximum  and 
decreases  to  zero,  while  the  active  electromotive  force  passes 
from  a  negative  maximum  to  a  positive  maximum.  In  other 
words,  while  the  active  electromotive  force,  and  therefore  the 
line  current  with  which  it  is  in  phase,  decreases  from  a  positive 
maximum  value  to  a  negative  maximum  value,  the  condenser 
current  is  negative,  and  while  the  active  electromotive  force 
increases  from  its  negative  to  its  positive  maximum  value  the 

*  The  active  electromotive  force  is  the  resultant  of  the  impressed  electromotive 
force  and  the  condenser  electromotive  force,  and  is  in  phase  with  the  current  actually 
flowing  in  the  line. 


132  AMERICAN   TELEPHONE   PRACTICE. 

condenser  current  is  positive.  The  condenser  current  therefore 
reaches  its  zero  value,  while  decreasing,  90°  in  advance  of  the 
same  value  of  the  active  electromotive  force ;  its  maximum 
negative  value  90°  in  advance  of  the  maximum  negative  value 
of  the  active  electromotive  force  ;  and  upon  investigation  it  will 
be  found  that  every  value  of  the  condenser  current  occurs  90° 
in  advance  of  the  corresponding  value  of  the  actual  line  current. 
The  electromotive  force  which  is  in  phase  with  the  condenser 
current  is  called  the  condenser  electromotive  force,  and  is  90°  in 
advance  of  the  electromotive  force  which  is  active  in  driving 
current  through  the  line.  This  latter  electromotive  force 
which,  as  we  have  said,  is  in  phase,  with  the  current  flowing 
in  the  line,  is  the  resultant  of  the  impressed  electromotive  force 
and  the  condenser  electromotive  force,  and  therefore  leads  the 
impressed  electromotive  force  by  a  certain  angular  distance. 

The  current  equation   for  a  circuit  containing  resistance  and 
capacity  is,  as  before, 

Electromotive  Force 
Current  =  - 


Impedance 

In  this  case  the  impedance  depends  on  the  ohmic  resistance 
of  the  circuit  and  on  its  capacity,  and  is  equal  to  the  following 
expression : 


R  + 


where  f  is  the  frequency,  as  before,  R  the  ohmic  resistance, 
and  C  the  capacity  of  the  condenser  in  farads.  From  this  the 
current  equation  becomes 

/=  .*- 


The  denominator  is  the  apparent  resistance,  depending  upon 
the  ohmic  resistance  of  the  circuit,  the  capacity,  and  the  frequency 
of  alternations.  An  inspection  of  this  equation  will  show  that  as 
the  frequency,  /,  is  increased  the  impedance  or  apparent  resist- 
ance becomes  smaller,  and  this  accounts  for  the  fact  that  a  con- 
denser will  readily  transmit  rapidly  fluctuating  currents,  such  as 
voice  currents.  Evidently  the  effect  of  increasing /"reduces  the 
second  member  in  the  denominator  of  the  equation,  and  if 


SELF-INDUCTION  AND    CAPACITY.  133 

sufficiently   great,    this   may   be    neglected,    and   the    equation 
becomes  simply 


Again,  increasing  the  capacity  of  the  condenser  also  increases 
the  effective  current  by  reducing  the  impedance. 

Every  telephone  line  possesses  electrostatic  capacity,  and  may 
be  considered  in  the  light  of  a  condenser.  In  the  case  of  a 
grounded  circuit  the  line  wire  takes  the  part  of  one  plate  of  the 
condenser,  while  the  ground  and  other  neighboring  conductors 
act  as  the  other  plate.  In  metallic  circuits  the  one  side  of  the 
line  acts  by  electrostatic  induction  upon  the  other,  and  the  two 
together  upon  the  ground  and  neighboring  conductors.  The 
capacity  of  a  line,  however,  differs  from  that  of  a  condenser  in 
that  it  is  distributed  throughout  the  entire  length  of  a  line,  while 
a  condenser  connected  with  a  line  would  have  all  of  its  capacity 
at  a  single  point.  Capacity  such  as  that  of  a  line  is  termed  dis- 
tributed capacity  in  order  to  distinguish  it  from  that  possessed 
by  a  condenser,  which  may  be  termed  local  capacity.  The 
effect  of  distributed  capacity  upon  telephone  currents  may  be 
more  readily  grasped  by  imagining  a  large  number  of  con- 
densers bridged  across  the  two  sides  of  a  metallic  circuit  at 
frequent  intervals.  When  the  electromotive  force  impressed 
upon  one  end  of  the  line  increases,  a  current  flows  from 
the  source  over  the  line  wire  and  into  the  condensers,  charging 
them  all  according  to  the  difference  of  potential  across  their 
terminals. 

The  difference  of  potential  across  the  terminals  of  all  the  con- 
densers will  not  be  the  same,  because  there  is  a  certain  drop, 
due  to  the  ohmic  resistance  of  the  line  wire.  If  the  current 
flows  in  that  direction  long  enough  to  charge  all  of  the  con- 
densers on  the  line  then  a  current  will  reach  the  distant  end  of 
the  line,  and  if  it  continues  in  that  direction,  will  attain  its  full 
value,  in  accordance  with  Ohm's  law.  It  must  be  remembered, 
however,  that  each  condenser  is  capable  of  taking  a  certain 
charge,  and  in  order  to  receive  this  charge  a  certain  quantity  of 
electricity  must  flow  over  the  line  wire. 

The  quantity  of  electricity  which  flows  through  a  circuit 
depends  upon  the  strength  of  the  current  and  upon  the  time 
it  is  allowed  to  flow.  If  the  time  is  insufficient  it  will  be  im- 
possible for  enough  current  to  flow  through  the  circuit  to  charge 
the  condensers  up  to  the  potential  of  the  source,  and  therefore 


134  AMERICAN    TELEPHONE   PRACTICE. 

the  current  at  the  distant  end  of  the  line  will  not  reach  its  maxi- 
mum value,  and  in  fact  may  not  rise  practically  above  zero.  If, 
therefore,  the  electromotive  force  of  the  source  is  reversed 
before  sufficient  current  has  time  to  pass  through  the  line  to 
charge  all  of  the  condensers,  the  current  will  not  reach  its  full 
value  at  the  distant  end  of  the  line. 

It  may  begin  to  build  up  in  the  opposite  direction,  and  again 
be  stopped  on  account  of  the  insufficient  time  to  reach  its 
proper  value  in  that  direction.  The  time  in  which  the  current 
in  such  a  circuit  will  reach  a  definite  portion  of  its  maximum 
value  at  the  distant  end  of  the  line  is  called  the  time  constant, 
and  if  the  time  represented  by  one  alternation  of  the  electro- 
motive force  is  smaller  than  the  time  constant,  the  current  will 
not  reach  that  value  at  the  distant  end  of  the  circuit,  and  the 
transmission  will  be  correspondingly  impaired. 

The  reduction  in  the  actual  volume  of  current  transmitted  by 
the  effects  of  distributed  capacity  is,  however,  of  less  importance 
than  the  distortion  of  the  wave  form.  The  higher  frequencies 
of  current  waves  corresponding  to  the  higher  overtones  are 
absorbed  by  the  condensers  far  more  readily  than  the  lower 
frequencies,  and  therefore  the  waves  corresponding  to  the  higher 
overtones  are  reduced  to  a  much  greater  extent  at  the  distant 
end  of  the  line  than  those  corresponding  to  the  fundamental 
and  the  lower  overtones.  This  weeds  out  the  upper  harmonics, 
thus  tending  to  destroy  the  clearness.  Capacity,  however,  acts 
in  still  another  way  to  alter  the  form  of  the  wave.  The  angle  of 
advance  for  the  higher  frequencies  is  greater  than  that  for  the 
lower,  and  therefore  the  waves  of  different  frequencies  are 
shifted  with  respect  to  their  phase  relation,  thus  greatly  altering 
the  wave  form. 

It  has  been  shown  that  the  electromotive  force  of  self-induc- 
tion lags  90°  behind  the  active  electromotive  force,  while  the 
electromotive  force  due  to  capacity  is  90°  in  advance  of  the 
active  electromotive  force.  It  is  not  difficult  to  conceive,  there- 
fore, that  by  properly  proportioning  the  self-induction  and 
capacity  of  a  circuit  the  electromotive  force  of  self-induction  may 
be  made  to  neutralize  the  electromotive  force  of  capacity,  and 
this  result  is  readily  obtained  in  experimental  work. 

In  this  case,  even  though  self-induction  and  capacity  may  be 
present  in  a  circuit  to  a  large  degree,  the  current  flowing  in  the 
circuit  is  in  exact  phase  with  the  impressed  electromotive  force, 
and  its  value  is  in  strict  accordance  with  the  ordinary  expression 
of  Ohm's  law.  Unfortunately,  however,  for  long-distance  teleph- 


SELF-INDUCTION  AND   CAPACITY.  135 

ony  such  a  balancing  of  self-induction  against  capacity  can  be 
obtained  only  for  one  particular  frequency  at  a  time.  To  thus 
tune  a  circuit  for  one  particular  frequency  would  render 
that  circuit  capable  of  transmitting  efficiently  one  particular 
frequency  of  vibration,  while  the  requirements  of  telephony  are 
that  all  frequencies  within  the  range  of  the  human  voice  shall  be 
transmitted  with  equal  facility.  'Again,  and  unfortunately,  it  has 
been  found  impossible  to  neutralize  distributed  capacity  with 
anything  but  distributed  self-induction,  and  this  has  not  yet 
been  accomplished  in  practice. 

As  for  trans-oceanic  telephony,  the  high  static  capacity  of  the 
cable  has  so  far  proven  an  insurmountable  obstacle.  It  is  im- 
possible to  conceive  a  transmitter  capable  of  forcing  such  rapid 
undulations  through  our  present  form  of  cables.  Clearly,  then, 
the  solution  lies  in  the  betterment  of  cables,  or  the  substitution 
of  some  other  transmission  medium,  rather  than  the  improve- 
ment of  the  instruments  themselves. 

There  can  be  little  doubt  that  trans-oceanic  telephony  will 
finally  be  accomplished,  but  the  indications  are  that  our  knowl- 
edge at  the  present  time  is  not  sufficient  to  cope  with  the. 
problem. 


CHAPTER  XIII. 

TELEPHONE    LINES. 

IN  the  early  days  of  telephony,  the  fact  discovered  by  Steinheil, 
that  the  earth  could  be  used  instead  of  the  return  wire  of  an 
electric  circuit,  was  made  use  of,  and  telephone  lines  were  gener- 
ally constructed  accordingly — that  is,  with  but  a  single  wire,  using 
the  earth  as  the  return. 

Lines  so  constructed  were,  however,  soon  found  to  be  subject 
to  serious  difficulties,  chief  among  which  were  the  strange  and 
unaccountable  noises  heard  in  the  receiving  instruments.  There 
are  many  causes  for  such  noises,  some  of  which  are  not  entirely 
understood.  The  swinging  of  the  wire,  in  such  manner  as  to  cut 
through  the  lines  of  force  of  the  earth's  magnetic  field,  or  the 
sudden  shifting  of  the  field  itself,  causes  currents  to  flow  in  the 
line  wire  which  may  produce  sounds  in  the  receiver.  On  long 
grounded  lines  the  variation  in  the  potential  of  the  earth  at  the 
ground  plates,  due  to  any  cause  whatever,  will  cause  currents  to 
flow  in  the  line.  The  passing  of  clouds  or  bodies  of  air  charged 
with  electricity  will  induce  charges  in  the  line,  and  cause  currents 
to  flow  to  or  from  the  earth  through  the  receiving  instruments. 
Electric  storms  and  auroral  displays  apparently  greatly  heighten 
these  effects.  These  noises  are  of  varying  character,  and  Mr.  J. 
J.  Carty  well  describes  them  in  saying: 

"  Sometimes  it  sounded  as  though  myriads  of  birds  flew  twitter- 
Ing  by  ;  again  sounds  like  the  rustling  of  leaves  and  the  croaking 
of  frogs  could  plainly  be  heard ;  at  other  times  the  noises  resem- 
bled the  hissing  of  steam  and  the  boiling  of  water." 

The  noises  due  to  these  natural  phenomena,  whatever  their 
true  cause  may  be,  are  chiefly  annoying  on  long  lines,  short  lines 
being  disturbed  only  during  heavy  electrical  storms.  This  is  not 
the  case,  however,  with  the  noises  arising  from  the  proximity  of 
other  wires  carrying  varying  currents.  Telegraphic  signals  can  be 
plainly  heard  in  a  telephone  instrument  on  a  line  running  par- 
allel with  a  neighboring  telegraph  line  for  a  very  short  distance. 
The  establishment  of  an  electric  railway  or  electric  lighting  plant 
in  a  town  using  grounded  telephone  lines  will  always  cause  seri- 
ous noises  in  the  telephones,  and  if  the  lighting  current  is  alter- 

136 


TELEPHONE  LINES. 


137 


nating  the  use  of  the  telephones  is  usually  out  of  the  question  at 
night  time,  while  the  plant  is  running. 

Disturbances  on  telephone  lines  from  neighboring  wires  may 
be  attributed  to  one  or  all  of  the  following  three  causes:  leakage, 
electromagnetic  induction,  and  electrostatic  induction. 

Leakage  may  occur  through  defective  insulation  between  the 
t\vo  circuits  ;  or  even  when  the  insulation  of  the  wires  themselves 
is  practically  perfect  a  heavy  return  current  from  a  grounded 
circuit,  such  as  of  an  electric  railway,  may,  upon  its  arrival  at  the 
grounded  end  of  the  telephone  line,  have  the  choice  of  two  paths, 
one  through  the  telephone  line,  and  the  other  a  continuation 
of  its  path  through  the  ground.  This  is  the  greatest  source 
of  trouble  due  to  railway  work,  on  grounded  telephone  lines. 
A  strange  fact  in  connection  with  this  is  that  the  noises  in  the 
telephones  do  not  correspond  with  the  fluctuations  due  to  the 
commutator  of  the  generator  armature,  as  would  be  supposed,  but 


Fig.  117.— Magnetic  Lines  around  a  Conductor. 

to  the  movements  of  the  armatures  on  the  car  motors.  The  tone 
in  the  receiver  is  an  indication  of  the  movements  of  the  car,  and 
variations  in  speed  may  be  clearly  noticed. 

Electromagnetic  induction  is  due  to  the  fact  that  the  telephone 
line  lies  in  the  field  of  force  set  up  by  the  disturbing  wire.  About 
every  wire  carrying  a  current  there  is  a  field  of  force,  or  "  magnetic 
whirl,"  consisting  of  closed  lines  of  force  surrounding  the  con- 
ductors. Such  a  condition  is  represented  in  Fig.  117.  If  the  curl 
rent  is  a  continuous  one,  the  lines  of  force  will  not  vary  after  being 
once  set  up,  and  the  telephone  wire  lying  in  this  field  will  not  be 


I38  AMERICAN    TELEPHONE   PRACTICE. 

affected.  If  the  current  in  the  disturbing  wire  is  fluctuating,  the 
number  of  lines  of  force  in  this  field  will  vary;  or,  by  a  clearer  way  of 
expressing  it,  the  field  of  force  will  expand  and  contract  accord- 
ingly. This  expansion  and  contraction  of  the  field  will  cause  its 
lines  of  force  to  cut  the  telephone  wire,  and  will  by  the  laws  of 
electromagnetic  induction  cause  currents  to  flow  in  the  latter. 
If  the  current  in  the  disturbing  wire  is  an  alternating  one,  the 
field  of  force  around  it  will  be  established  in  one  direction,  de- 
stroyed and  established  in  the  reverse  direction,  and  again 
destroyed,  with  every  complete  cycle  of  the  current.  It  is  easy 
to  see  that  this  will  produce  a  maximum  disturbance  in  the  tele- 
phone wire. 

Electrostatic  induction  may  be  explained  by  reference  to  Fig. 
1 1 8,  where  a  grounded  telephone  line  is  shown  running  parallel 
with  a  disturbing  wire,  which  we  will  say  is  carrying  an  alternat- 
ing electric  current.  The  disturbing  wire  will  receive  from  its 
source  of  current  alternate  positive  and  negative  charges  of 
electricity,  and  its  potential  will  pass  from  a  maximum  in  one 

DISTURBING  WIRE 
+     + +    -J-     4-     4-     4      + + +      £ +•     + + + ^ + 


Fig.  118. — Electrostatic  Induction. 

direction  through  zero  to  a  maximum  in  the  other,  and  again 
through  zero  to  the  maximum  in  the  first  direction  during 
each  cycle. 

Consider  the  condition  where  the  potential  of  the  disturb- 
ing wire  is  zero.  No  charge  will  then  be  induced  on  the  tele- 
phone wire,  so  that  its  potential  will  also  be  zero.  The 
charge  on  the  disturbing  wire  then  becomes,  we  will  say,  posi- 
tive, and  this  induces  a  bound  negative  charge  on  the  side  of  the 
telephone  wire  nearest  the  disturbing  wire,  and  an  equal  posi- 
tive charge  on  the  opposite  side.  This  latter  charge  is  not 
bound,  and  flows  to  earth  through  the  receivers  at  each  end. 
This  flow  will  be  toward  the  ground,  through  each  receiver,  and 
the  current  is  therefore  from  the  center  of  the  wire  in  each 
direction  to  the  ground.  The  next  instant  the  potential  of  the 
disturbing  wire  becomes  zero,  thus  relieving  the  bound  negative 
charge  on  the  telephone  wire,  which  flows  to  earth,  or,  more 
properly,  is  neutralized  by  a  flow  of  positive  electricity  from  the 


TELEPHONE  LINES.  139 

earth.  Thus  each  change  in  potential  of  the  disturbing  wire 
causes  a  flow  of  current  through  the  receivers  at  each  end,  this 
flow  always  being  toward  or  from  the  middle  point  in  the  length 
of  the  wire.  These  currents  produce  noises  in  the  receivers  at 
each  end  in  the  ordinary  way. 

When  two  grounded  telephone  circuits  run  side  by  side,  each 
acts  inductively  on  the  other,  so  that  a  conversation  carried  on 
over  one  circuit  may  be  heard  in-  the  telephones  on  the  other. 
This  phenomenon  is  aptly  termed  cross-talk,  and  is  usually  ex- 


4-      4- 


Fig.  119. — Electrostatic  Induction. 

plained  in  text-books  and  articles  on  the  subject  by  the  sup- 
position that  it  is  chiefly  if  not  entirely  due  to  electromagnetic 
induction. 

In  1889,  however,  Mr.  J.  J.  Carty,  in  a  paper  before  the  New 
York  Electric  Club,  and  again  in  1891,  in  another  paper  before 
the  American  Institute  of  Electrical  Engineers,*  described  a 
series  of  experiments  which  show  conclusively  that  cross-talk 
between  lines  is  due  almost  entirely  to  electrostatic  induction, 
electromagnetic  induction  playing  so  small  a  part  as  not  to  be 
noticeable. 

The  arrangement  of  circuits  in  one  of  his  experiments  is 
shown  in  Fig.  119,  in  which  E  .Fatid  CD  are  two  well-insulated 
lines,  each  200  ft.  long,  and  placed  parallel  with  each  other 
throughout  their  entire  length,  at  a  distance  of  \  in.  apart.  E  F 
is  the  disturbing  line  and  is  left  open  at  E.  At  F  it  is  connected 
through  the  secondary  of  an  induction  coil,  Z,  with  the  ground. 
In  the  primary  circuit  of  this  coil  is  a  battery,  B,  and  a  Blake 
transmitter,  T.  A  tuning  fork  vibrating  before  the  transmitter 
acted  on  the  diaphragm  in  the  usual  way,  and  caused  impulses 
on  the  line  E  F  of  practically  the  same  strength  as  voice  cur- 
rents. These  impulses  are,  of  course,  alternately  positive  and 
negative,  and  may  be  considered  in  the  same  light  as  the 
impulses  on  the  disturbing  line  in  Fig.  118.  Three  receivers, 

*  These  papers  should  be  read  by  all  interested  in  this  subject. 


140  AMERICAN    TELEPHONE  PRACTICE. 

x,y,  and  z,  were  placed  in  the  line  C  D,  the  receiver,  y,  being  at 
the  middle  point  in  the  line.  Upon  operating  the  tuning  fork, 
its  musical  note  could  be  distinctly  heard  in  receivers,  x  and  z, 
while  y  remained  silent. 

In  explaining  the  action  of  static  induction  in  connection  with 
Fig.  1 1 8,  it  was  pointed  out  that  the  flow  of  induced  currents 
would  be  either  toward  or  from  the  middle  point  in  the  length 
of  the  wire.  The  silence  of  the  receiver,  y,  in  this  case  bears  out 
that  statement,  showing  the  central  point  to  be  neutral.  If  this 
were  electromagnetic  induction,  the  induced  current  would  pass 
from  one  end  of  the  line,  CD,  to  the  other,  returning  through 
the  ground,  in  which  case  all  the  receivers  would  be  affected. 
As  it  is,  however,  the  induced  charges  flow  in  each  direction 
from  the  receiver,  y,  to  the  ground  at  each  end,  or  from  the 
ground  at  each  end  to  the  receiver,  y,  thus  in  no  case  causing 
its  diaphragm  to  vibrate.  The  same  results  were  obtained  by 
grounding  the  point  E  through  an  ordinary  telephone.  The 
receiver  wire  still  remained  silent,  while  x  and  z  were  both 
affected  to  an  equal  degree. 

It  was  also  found  that  opening  the  central  point  of  the  line, 


DISTURBING  WIRE 


B 
Fig.  120. — Electromagnetic  Disturbances. 

C  D,  produced  no  effect  whatever  on  the  existing  conditions  ; 
the  noises  in  the  receivers,  x  and  zt  were  plainly  heard  and  of 
equal  loudness. 

Many  other  experiments  were  tried,  the  results  in  each  case 
pointing  conclusively  to  the  induction  from  voice  currents  being 
of  an  electrostatic  instead  of  an  electromagnetic  nature. 

There  is  no  doubt,  however,  but  that  induction  from  wires 
carrying  heavy  currents,  such  as  are  used  in  lighting  and  power 
work,  is  largely  due  to  electromagnetic  effects,  and  this  can 
be  easily  proven  by  experiments  similar  in  nature  to  those 
described. 

The  one  remedy  for  all  the  troubles  due  to  disturbing  noises 
from  any  of  the  causes  is  to  make  the  line  a  complete  metallic 
circuit.  Even  this  will  not  completely  stop  noises  from  most  of 
the  causes,  and  all  additional  precaution  must  be  taken,  by  mak- 
ing the  two  sides  of  the  circuit  alike  in  all  respects  and  properly 


TELEPHONE   LINES.  141 

transposing  them  at  frequent  intervals,  in  order  that  they  may  be 
as  nearly  symmetrical  with  respect  to  the  disturbing  source  or 
sources  as  possible. 

Merely  making  the  line  a  metallic  circuit,  as  in  Fig.  120,  does 
not  give  complete  freedom  from  inductive  troubles  from  other 
wires,  whether  the  induction  be  electromagnetic  or  electrostatic. 
Considering  the  question  from  the  standpoint  of  electromagnetic 
induction,  a  current  flowing  in  the  disturbing  wire  would  set  up  a 
field  of  force,  the  lines  of  which  would  cut  conductors,  A  and  B. 
A  being  closer,  however,  would  be  cut  by  more  lines  than  B,  and 
consequently  any  currents  induced  in  A  by  changes  in  this  field 
will  be  stronger  than  those  in  B.  If  a  current  starts  to  flow  in 
the  disturbing  wire  from  right  to  left,  as  shown,  the  induced  cur- 
rents in  A  and  B  will  each  be  from  left  to  right,  as  indicated  by 
the  arrows.  These  currents  will  partially  annul  each  other,  but 
that  in  A,  being  the  stronger,  will  predominate,  and  the  resultant 
will  flow  in  the  circuit  in  a  direction  indicated  by  the  small  curved 
arrows. 

A  single  transposition  in  the  center  of  the  metallic  circuit  will 
completely  annul  the  electromagnetic  induction  if  the  disturbing 
wire  is  parallel  to  the  two  wires  throughout  its  entire  length,  and 
if  it  carries  the  same  current  in  all  its  portions.  Here  an  impulse 
in  the  direction  of  the  arrow  in  the  disturbing  wire  (Fig.  121)  will 


DISTURBING  WIRE 


B  A 

Fig.  121. — Electromagnetic  Disturbances. 

cause  impulses  in  the  opposite  direction  in  both  wires,  A  and  B. 
As  the  average  distances  between  the  disturbing  wire  and  A  and 
B,  respectively,  are  the  same,  the  strengths  of  the  induced  currents 
in  A  and  B  will  be  equal,  and  they  will,  therefore,  annul  each 
other,  producing  no  sound  in  the  receivers. 

It  is  found,  however,  that  a  single  transposition  in  the  center 
of  the  metallic  circuit  will  not  free  the  line  from  cross-talk,  even 
though  the  average  distance  from  the  two  wires  and  the  dis- 
turbing wire  is  the  same,  and  the  current  strength  is  uniform 
throughout  the  entire  length  of  the  disturbing  wire. 

Mr.  Carty's  experiments  throw  much  light  on  this  point.  In 
Fig.  122  is  shown  a  disturbing  wire  and  a  metallic  telephone  cir- 
cuit composed  of  two  wires,  A  and  B,  of  which  A  is  nearer  the 


I42  AMERICAN    TELEPHONE   PRACTICE. 

disturbing  wire  than  B.  At  a  time  when  the  charge  on  the  dis- 
turbing wire  is  positive,  as  shown,  a  negative  charge  will  be  drawn 
by  it  toward  the  disturbing  wire  and  a  positive  charge  will  be 
repelled  from  it.  The  result  is  that  the  distribution  of  charges 
on  the  two  wires,  A  and  B,  will  be  somewhat  as  shown,  a  nega- 
tive charge  being  held  on  the  wire,  A,  and  a  positive  charge  driven 
to  the  wire,  B. 

In  order  for  this  rearrangement  to  have  occurred,  it  is  evident 
that  a  flow  of  electricity  must  have  taken  place  from  A  to  B,  and 
as  two  paths  were  afforded  from  the  center  point,  #,  on  the  wire 
A,  of  equal  resistance,  this  flow  must  have  been  from  that  point 
in  each  direction  as  indicated  by  the  arrows,  through  the  receivers 
and  toward  the  center  point,  b,  on  wire,  B,  where  the  two  currents 
met.  Upon  the  charge  on  the  disturbing  wire  becoming  zero  the 
potentials  on  A  and  B  become  equal,  by  a  flow  of  positive  elec- 
tricity from  the  center  point  of  wire,  B,  to  that  of  wire,  A.  The 
negative  charge  on  the  disturbing  wire,  which  follows  the  positive 

DISTURBING  WiRE 


B  6 

Fig.  122. — Electrostatic  Disturbances. 

charge,  will  cause  this  latter  to  flow  from  bio  a,  to  continue  until 
A  is  positively  and  B  negatively  charged. 

It  is  evident,  therefore,  that  alternating  currents  flow  through 
two  receivers,  and  that  these  currents  differ  in  phase  from  that 
in  the  disturbing  wire  by  90  degs.,  which  is  characteristic  of  the 
action  of  condensers.  Further  consideration  will  show  that  the 
points  a  and  b  are  neutral,  and  experiment  bears  out  this  conclu- 
sion, for  by  opening  the  wires  at  those  points  the  sound  in  the 
receivers  at  the  ends  still  continues. 

Where  receivers  are  connected  in  the  circuit  at  a  and  b  no 
sound  is  heard  on  them,  although  plainly  audible  in  the  end 
receivers.  A  single  transposition  in  the  center  of  the  line,  as 
shown  in  Fig.  123,  will  tend  to  reduce  the  sound  in  the  end 
receivers,  but  will  not  cause  silence.  The  static  charges  on  the 
portions  of  the  wires  nearest  to  the  disturbing  wire  now  find  four 
paths  instead  of  two  to  the  more  remote  portions  of  the  circuit, 
the  flow  being  clearly  indicated  by  the  arrows.  The  center  points, 
a  and  b,  are  no  longer  neutral,  and  receivers  placed  in  the  circuit 
there  will  be  subject  to  noises. 


TELEPHONE   LINES. 


It  is  evident  that  if  receivers  of  equal  impedance  to  those  at  the 
ends  of  the  line  were  placed  at  a  and  b,  the  neutral  points,  c,  d,  e, 
and/,  would  be  found  at  the  quarter  points  on  the  line  ;  i.  e.,  mid- 
way between  the  transposition  and  each  end.  As  a  matter  of 
fact,  however,  no  instruments  are  placed  at  the  point  of  trans- 
position, and  the  neutral  points  are  shifted  toward  the  ends  of  the 
line,  because  the  impedance  of*  the  receivers  at  those  points 


-4-        4-        + 


DISTURBING  WIRE 

-h  4-4-4- 


H-      4- 


(> 

> 


-I-    + 


A' 


/ 


Fig.  123. — Electrostatic  Disturbances. 

makes  it  easier  for  most  of  the  current  to  pass  through  the  trans- 
position wires. 

Theoretically,  the  currents  set  up  in  a  metallic  circuit  by 
electrostatic  induction  from  another  circuit  can  be  eliminated 
only  by  making  an  infinite  number  of  transpositions.  Practi- 
cally, however,  it  is  found  that  on  long  circuits  transpositions 
every  quarter-  or  half-mile  are  amply  sufficient  to  render  them 
unnoticeable. 

The  scheme  of  transposition  used  by  the  American  Telegraph 
and  Telephone  Company  on  the  New  York-Chicago  telephone  line 
is  shown  in  Fig.  124.  It  will  be  seen  from  this  figure  that  trans- 


#—  /300/+- 

f-jjoo/e  1 

Y  1300ft- 

'S/ipercr 

QSS  Jlrn 

'V 

j 

(^ 

/ 

\ 

\ 
/ 

\ 

m 

ver-  Cr 

?5s  Arr 

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1 

Ji. 

) 

) 

( 

i 

a  ' 

Jto  

^ 

Fig.  124. — Diagram  of  Transpositions. 

positions  are  made  on  this  line  practically  four  times  in  every 
mile,  that  is,  upon  every  tenth  pole;  and  while  this  involves  the 
placing  of  transposition  insulators  on  poles  a  quarter  of  a  mile 


144  AMERICAN    TELEPHONE  PRACTICE. 

apart,  it  does  not  follow  that  every  circuit  is  transposed  at  each  of 
these  intervals.  The  reason  for  this  arrangement  is  that  if  two 
lines  running  side  by  side  were  transposed  in  exactly  the  same 
manner  throughout  their  lengths,  the  desired  non-inductive  condi- 
tion would  not  be  secured  for  the  relation  between  the  corre* 
spending  wires  in  the  two  circuits  would  then  be  the  same  as  if  no 
transpositions  whatever  had  been  made.  In  order  to  overcome 
this  difficulty,  transpositions  on  the  second  circuit  should  be 
made  twice  as  often  as  those  on  the  first.  This  is  the  scheme 
adopted  in  Fig.  124,  where  it  will  be  seen  that  the  center  pair  of 
wires  on  each  set  of  cross-arms  is  transposed  every  mile,  while 
the  pair  immediately  adjacent  to  it  on  each  side  is  transposed 
twice  as  often.  The  outside  pairs  on  each  cross-arm  are  trans- 
posed only  once  per  mile,  but  these  transpositions  are  staggered 
with  respect  to  those  on  the  center  pair.  The  same  scheme  is 
followed  out  on  every  cross-arm,  but  the  transpositions  on  the 
top  set  of  cross-arms  are  staggered  with  respect  to  those  on  the 
set  immediately  below — this  being  the  case  throughout  the 
entire  number  of  cross  arms  on  a  pole  ;  the  1st,  3d,  5th,  /th,  and 
9th  being  transposed  according  to  the  scheme  shown  in  the  upper 
part  of  Fig.  124,  while  the  circuits  on  arms  Nos.  2,  4,  6,  8,  and  10 
are  transposed  according  to  the  scheme  in  the  lower  part  of  this 
figure. 

A  very  perfect  transposition  is  effected  by  twisting  two  sides  of 
a  circuit  together,  and  this  idea  is  followed  out  in  the  English 
pole-line  construction,  where  the  two  sides  of  the  circuit  are  not 
only  transposed  laterally,  but  also  pass  successively  over  and  under 
each  other  several  times  in  each  mile,  thus  effectually  giving  the 
circuit  a  number  of  complete  twists.  This  method,  however, 
involves  several  disadvantages  in  the  stringing  of  wires,  and 
increases  the  liability  of  crosses  between  them.  It  is  not,  there- 
fore, adopted  to  any  considerable  degree  in  this  country. 

The  twisted  pair  of  insulated  wires  used  so  largely  in  inside 
wiring,  and  also  in  cable  work,  accomplishes  the  transposition  of 
circuits  very  thoroughly,  it  in  fact  amounting  to  a  complete 
transposition  for  every  twist  of  the  wires.  This  method  is  now 
depended  upon  entirely  in  the  construction  of  telephone  cables, 
with  so  great  a  degree  of  success  as  to  absolutely  prevent  all 
induction  between  the  circuits.  This  will  be  discussed  at  greater 
length  in  the  chapter  on  cables. 

Where  a  number  of  lines  radiate  from  a  central  point  to  a 
number  of  subscribers'  stations  the  cheapest  way  of  arranging 
the  circuits,  if  expense  alone  is  to  be  considered,  is  to  make  each 


TELEPHONE  LINES.  145 

a  grounded  circuit.  This  is  done  by  grounding  each  line  at  the 
subscribers'  station  after  it  has  passed  through  the  telephone 
there,  and  also  at  the  central  office  after  it  has  passed  through 
the  coil  of  the  annunciator  or  signaling  device.  Such  an  arrange- 
ment is  shown  in  Fig.  125,  and  it  may  be  assumed  that  the  lines 
there  shown  run  in  the  same  direction — on  the  same  poles  or  in 
the  same  cable — to  the  various  subscribers  whom  they  serve.  In 
each  case  D  is  the  line  drop  at  the  central  office  and  R  repre- 
sents the  entire  telephone  set  at  the  several  subscribers'  stations. 
It  is  evident  that,  with  such  an  arrangement,  disturbances  in 
the  receivers  may  be  produced  by  any  one  or  all  of  the  causes 
already  considered.  An  electric  light  or  power  wire,  carrying  a 


CENTRAL 
OFFICE 


Fig.  125. — Ground-Return  Systems. 

heavy  current,  may  cause  trouble  by  electrostatic  or  electro- 
magnetic induction,  or  by  leakage  ;  and,  moreover,  each  telephone 
wire,  when  in  use,  may  be  a  disturbing  wire  to  all  of  the  others. 

As  has  been  pointed  out,  the  proper  remedy  for  these  disturb- 
ances is  to  make  each  line  a  separate  metallic  circuit,  and  to  prop- 
erly transpose  the  two  sides  of  each  circuit  at  frequent  intervals 
where  the  lines  are  long.  This  course  is  followed  in  most  large 
telephone  exchanges,  and  many  small  ones  ;  but  it  frequently 
happens  that  commercial  considerations  will  not  allow  it  in 
smaller  installations.  Where  this  is  the  case  a  system  called  the 
common-return  or  McCluer  system  is  frequently  used,  with  ex- 
cellent results.  The  layout  is  the  same  as  that  of  the  grounded 
system,  with  the  exception  that  the  return  of  every  circuit  is 
made  through  a  heavy  wire  common  to  all  of  the  circuits.  A 
clear  conception  of  the  common-return  system  may  be  had  by 
considering  a  heavy  wire,  Fig.  126,  to  take  the  place  of  the  earth 
in  a  grounded  system  ;  each  line  wire  being  connected  to  it  at  or 
near  the  subscriber's  station  after  passing  through  the  telephone 


i46 


AMERICAN   TELEPHONE  PRACTICE. 


instrument,  R,  and  at  the  exchange  after  passing  through  the 
switch-board  drop,  D. 

It  is  quite  evident  that  the  common-return  system  will,  if 
properly  installed,  remove  all  trouble  due  to  leakage  or  earth 
currents  ;  for  the  entire  system  of  wiring  may  be  kept  highly 
insulated  from  the  ground  and  from  other  conductors.  Practice, 
however,  differs  to  a  large  extent  in  this  respect,  as  some  com- 
panies ground  the  common-return  wire  at  the  exchange,  and  also 
at  several  other  points  along  its  length,  others  at  the  exchange 
only,  while  others  keep  it  entirely  insulated  from  earth.  Prob- 
ably the  reason  for  placing  several  grounds  on  the  return  wire  is 
to  effect  a  reduction  in  the  resistance  of  the  return  path,  but  this, 
if  needed,  should  be  brought  about  in  another  way.  Probably 
the  best  practice  in  most  cases  is  to  keep  entirely  free  from 
grounds,  although  there  are  many  who  claim  to  have  effected  a 
marked  improvement  by  heavily  grounding  the  common  return 
at  the  central  office.  At  any  rate,  this  is  an  easy  experiment  to 
try.  The  location  of  the  common-return  wire  with  respect  to  the 


COMMON  RETURN 

Fig.  126. — Common-Return  Systems. 

other  wires  on  the  poles  is  a  question  concerning  which  there 
is  much  difference  of  opinion.  Undoubtedly  the  best  way  of 
disposing  it,  so  far  as  purely  electrical  considerations  are  con- 
cerned, is  to  place  it  on  brackets  between  the  two  middle  cross- 
arms  on  the  poles,  for  then  it  bears  a  more  symmetrical  position 
with  respect  to  all  of  the  line  wires  and  is,  therefore,  better 
adapted  to  neutralize  induction  from  outside  sources.  It  is,  how- 
ever, often  put  on  other  parts  of  the  pole,  sometimes  above  all 
the  wires,  sometimes  on  a  bracket  just  below  the  top  cross-arm, 
and  frequently  below  the  lowest  cross-arm.  The  latter  is  prob- 


TELEPHONE  LINES.  147 

ably  the  most  convenient  place,  as  wires  led  off  to  buildings  will 
usually  stand  clear  of  the  other  wires  on  the  pole. 

There  is  even  more  difference  of  opinion  as  to  the  proper  size 
of  the  common-return  wire  than  as  to  its  location.  An  analysis 
of  the  inductive  action  from  neighboring  wires  may  perhaps 
throw  some  light  upon  the  subject.  If  we  assume  that  the  entire 
system  is  insulated  from  the  ground  and  from  other  conductors, 
it  will  be  safe  to  say  that  all  disturbances  in  the  telephones  due 
to  leakage  or  earth  currents  will  be  eliminated.  The  only  way, 
therefore,  by  which  neighboring  wires  can  affect  the  telephone 
wires  is  by  induction,  and  this  may  be  either  electromagnetic  or 
electrostatic,  or  both.  The  ideal  arrangement  of  the  common- 
return  wire  with  respect  to  the  line  wires  would  be  that  in  which 
all  were  at  an  equal  distance  from  the  disturbing  wire.  This  con- 
dition can  only  be  roughly  approximated  in  practice,  but  in  Fig. 
127  it  will  be  assumed  that  the  disturbing  wire,  which  may  be  a 


CENTRAL 
OFFICE 

u                                                 I 

A 

rc             2 

«==!* 

D 

IAAA/  c.  o.  DROP 

1                                 < 

S.R.                   b 

C' 


DISTURBING  WIRE 


Fig.  127. — Induction  on  Common-Return  Lines. 

trolley  or  electric  light  line,  is  at  approximately  the  same  dis- 
tance from  the  line  wires,  I  and  2,  from  the  return  wire,  C  R.  The 
lines,  i  and  2,  are  connected  at  central  by  the  cord  circuit, 
C,  a  clearing-out  drop  being  bridged  between  the  cord  and  the 
common  return,  and  the  line  drops,  D  D,  cut  out  as  in  ordinary 
practice. 

It  is  evident  that  if  no  sounds  are  to -be  produced  in  the 
receiver  of  Line  I,  the  points,  A  and  B,  must  be  of  equal  potential. 
Similarly  the  points,  a  and  b,  must  be  of  equal  potential  for  no 
disturbance  to  be  produced  in  the  receiver  of  Line  2,  and  the 
points,  C  and  C\  for  no  current  to  flow  through  the  clearing-out 
drop.  It  matters  not  if  C,  a,  and  A  are  all  of  different  poten- 
tials, the  one  requisite  for  silence  in  the  receivers  being  that  the 


148  AMERICAN   TELEPHONE  PRACTICE. 

two  points  at  the  terminals  of  each  shall  be  of  the  same  potential 
The  question,  therefore,  becomes,  what  size  of  wire  shall  be  used 
for  the  common  return  in  order  to  bring  about  these  conditions  ? 

Considering  the  induction  from  the  disturbing  wire  to  be  elec- 
tromagnetic, it  is  evident  that  the  electromotive  force  set  up  in 
the  several  wires  will  be  proportional  only  to  their  lengths,  as 
their  distances  from  the,  disturbing  wire  is  assumed  to  be  the 
same  in  each  case.  The  sizes  of  the  wire  will  have  nothing  to  do 
with  the  pressure  developed,  any  more  than  the  size  of  the  wire 
per  se  affects  the  electromotive  force  developed  in  the  armature 
winding  of  a  dynamo.  In  the  case  of  the  wire,  i,  the  E.  M.  F. 
generated  in  the  length,  C  A,  will  equal  that  generated  in  the 
length,  Cl  B,  of  the  common-return  wire.  This  means  that  the 
points,  A  and  B,  have  the  same  potential  as  have  also  C  and  C1t 
and  no  current  will  flow  through  the  receiver  of  that  line  or  the 
clearing-out  drop.  By  the  same  reasoning  the  pressure  devel- 
oped in  length,  C  a,  of  Line  2  will  equal  that  in  the  length,  C1  b, 
of  the  common  return,  and  no  current  will  flow  through  the 
receiver  of  Line  2.  So  far  as  electromagnetic  induction  is  con- 
cerned, therefore,  the  size  of  the  common-return  wire  is  im- 
material. 

Considering  the  question  from  the  standpoint  of  electro- 
static induction,  it  will  be  seen  that  for  a  given  charge  on'  the 
disturbing  wire  charges  of  the  same  potential  will  be  induced  on 
each  of  the  line  wires  and  on  the  common-return  wire.  There 
will,  perhaps,  be  a  tendency  for  the  small  wires  to  assume  charges 
of  slightly  higher  potential  than  the  larger  wire,  on  account  of 
their  smaller  radius  of  curvature,  but  this  would  be  so  slight  as 
to  be  negligible  under  the  conditions  assumed,  and  would  be 
eliminated  were  the  common-return  wire  made  of  the  same  size 
as  the  line  wires. 

So  far  as  inductive  disturbances  from  outside  sources  are 
concerned,  it  seems  that  the  size  of  the  common-return  wire  is 
practically  immaterial,  with  perhaps  a  slight  theoretical  advan- 
tage to  be  gained  by  making  it  of  the  same  size  as  the  line  wires. 
This  view  seems  to  be  at  direct  variance  with  nearly  all  written 
statements  on  the  subject. 

The  same  reasoning  will  show  that  when  any  one  of  the  tele- 
phone lines  is  considered  as  the  disturbing  wire,  the  same  conclu- 
sions are  reached. 

So  far  no  valid  reason  has  been  shown  for  making  the  com- 
mon-return larger  than  the  line  wires.  There  is,  however,  a 
good  reason  why  this  should  be  done,  and  this  is  the  fact  that 


TELEPHONE  LINES.  149 

the  return  circuit  of  any  one  wire  is  made,  not  only  through  the 
common-return  wire,  but  also  through  all  of  the  other  line  wires 
in  multiple.  Referring  to  Fig.  126,  it  is  evident  that  currents 
generated  in  line,  B  C,  may  find  a  return  path  through  the  com- 
mon-return wire,  and  through  all  of  the  other  wires  in  multiple. 
In  fact,  all  of  these  return  paths  will  be  chosen,  the  current 
dividing  among  the  other  line  wires  and  common  return,  in- 
versely as  to  their  respective  impedances. 

The  currents  flowing  through  these  other  lines  would  produce 
cross-talk  were  they  of  sufficient  magnitude  to  do  so,  and  the 
only  way  of  preventing  this  is  to  make  the  common-return  wire 
of  such  low  resistance  that  practically  all  of  the  current  will  pass 
through  it.  The  fact  that  the  common-return  wire  may  be  made 
to  possess  practically  no  self-induction,  and,  therefore,  only  the 
impedance  due  to  its  ohmic  resistance ;  while  the  line  wires  all 
include  in  their  circuits  either  the  receiver  and  induction  coils  or 
the  bell  and  drop  coils,  serves  to  divert  nearly  all  of  the  current 
through  the  common-return  wire,  where  it  belongs.  On  account 
of  the  marvelous  sensitiveness  of  even  poor  receivers,  a  compara- 
tively small  resistance  in  the  common  return  will  shunt  enough 
current  through  the  various  instruments  to  cause  cross-talk  to  a 
considerable  extent. 

There  is  undoubtedly  a  large  amount  of  copper  wasted  in 
common-return  wires,  and  it  is  probable  that  a  No.  8  B.  &  S. 
gauge  copper  wire  will  in  most  cases  answer  the  purpose.  It  too 
frequently  happens  that  larger  common-return  wires  are  used  in 
the  hope  of  remedying  a  difficulty  which  is  due  to  another  cause 
entirely.  Cross-talk  in  switch-boards  and  office  cables  is  often 
attributed  to  the  smallness  of  the  return  wire,  which  in  this 
case  might  be  increased  indefinitely  without  improving  the 
service. 

Of  course,  the  ideal  conditions  assumed  for  the  disposition  of 
the  wires  cannot  be  attained,  and  in  so  far  as  they  are  not 
attained,  induction  is  likely  to  occur. 

It  frequently  becomes  desirable  to  connect  a  grounded  line 
with  a  metallic  line,  and  for  this  purpose  what  is  known  as  the 
repeating  coil  forms  the  most  ready  solution.  A  repeating  coil  is 
merely  a  special  form  of  induction  coil,  usually  constructed  in  the 
form  shown  in  Fig.  128.  The  primary  and  secondary  coils  are 
wound  upon  a  heavy  core  formed  of  a  bundle  of  soft-iron  wires, 
after  which  the  ends  of  the  cores  are  bent  around  the  outside  of 
the  coil,  thus  completely  inclosing  it  in  a  casing  of  wire.  The 
coil  is  then  clamped  to  the  base,  usually  by  metal  straps,  as 


15°  AMERICAN   TELEPHONE  PRACTICE. 

shown  in  the  figure.  The  terminals  of  the  primary  are  brought 
out  to  binding  posts  at  one  end  of  the  base,  while  those  of  the 
secondary  are  similarly  brought  out  to  binding  posts  at  the  op- 
posite end.  The  wire  forming  the  core  should  be  of  No.  24 


Fig.  128. — Repeating  Coil. 

annealed  iron  formed  into  a  bundle  about  f  of  an  inch  in  diameter. 
The  two  coils  are  usually  made  equal  in  resistance  and  in  number 
of  turns,  200  ohms  for  each  coil  being  perhaps  the  most  common 
figure.  No.  31  B.  &  S.  gauge  silk-covered  wire  is  a  suitable  size 
for  a  coil  adapted  to  ordinary  work.  Of  course  these  figures  may 
be  departed  from  to  almost  any  degree  in  order  to  design  coils 
for  special  work. 

In  Fig.  129  is  shown  in  diagram  a  metallic  circuit  line   con- 
nected with  a  grounded  line  through  a  repeating  coil.     The  two 


METALLIC  CIRCUIT  /~5&\  GROUNDED  LIN 


•Q 

Fig.  129. — Connection  of  Metallic  and  Grounded. 

terminals  of  the  metallic  circuit  line  are  merely  brought  to  the 
two  binding  posts  of  one  of  the  coils,  while  the  terminal  of  the 
grounded  line  is  brought  to  one  of  the  binding  posts  of  the  other 
coil ;  the  remaining  binding  post  is  then  grounded.  Any  vary- 
ing currents  set  up  in  one  of  the  circuits  will  act  inductively  on 
the  other  circuit  through  the  windings  of  the  coil,  each  of  which 
may  thus  be  called  upon  to  act  alternately  as  a  primary  and  a 


TELEPHONE  LINES.  151 

secondary.  By  the  use  of  the  repeating  coil  in  this  manner, 
two  lines  may  be  connected  for  conversation  without  grounding 
one  side  of  the  metallic  circuit,  which  would  be  necessary  were 
the  repeating  coil  not  used. 

There  is  a  very  common  impression  among  the  independent 
telephone  users  that  a  repeating  coil  is  the  one  panacea  for  all 
of  the  evils  connected  with  grounded  lines.  It  is  perhaps  well  to 
correct  this  impression,  by  saying  that  no  number  of  repeating 
coils  will  render  a  noisy  grounded  line  quiet.  A  repeating  coil 
will,  however,  prevent  the  unbalancing  of  a  metallic  circuit  line, 
and  therefore  in  many  cases  insure  a  degree  of  quietness  on 
two  connected  lines  which  would  otherwise  be  unattainable. 

It  sometimes  happens  that  a  long  grounded  line  is  paralleled 
throughout  a  portion  of  its  length  only  by  some  disturbing  wire, 


DISTURBING  WIRE 


\ 


Fig.  130. — Eliminating  Local  Induction. 

such,  for  instance,  as  an  electric-light  line.  Where  it  is  not  pos- 
sible from  commercial  considerations  to  make  the  entire  line  a 
metallic  circuit,  much  relief  may  sometimes  be  had  by  resorting 
to  the  plan  shown  in  Fig.  130,  which  consists  in  making  only 
that  portion  of  the  line  a  metallic  circuit  which  is  within  the 
direct  influence  of  the  disturbing  wire.  The  two  ends  of  the 
grounded  line  may  then  be  connected  with  the  intermediate 
metallic  portion  by  means  of  the  repeating  coils,  R  R,  as  shown. 
By  this  arrangement  the  disturbing  wire  produces  no  effect  on 
the  metallic  circuit  between  the  repeating  coils,  if  proper  precau- 
tions are  taken  in  the  way  of  transposing  its  two  sides.  Tele- 
phonic communciation  may  be  had  over  the  entire  length  of  line, 
the  currents  undergoing  two  transformations  at  the  repeating 
coils. 

Much  trouble  is  often  had  where  it  is  necessary  to  ring 
through  repeating  coils,  especially  if  the  lines  are  very  long.  It 
is  therefore  advisable  that  repeating  coils  should  always  be 
placed  at  a  central  station  if  possible,  and  such  arrangements 


152  AMERICAN   TELEPHONE  PRACTICE. 

made  that  it  will  not  be  necessary  to  ring  through  them.  How- 
ever, a  coil  properly  constructed  with  a  magnetic  circuit  com- 
pletely closed  should  serve  as  a  very  efficient  transmitter,  even 
for  the  slowly  alternating  currents  of  a  magneto-generator,  and 
good  results  may  be  obtained  with  such  coils  on  good  lines  even 
when  it  becomes  necessary  to  ring  through  them. 


CHAPTER  XIV. 

SIMPLE   SWITCH-BOARD^   FOR   SMALL   EXCHANGES. 

THE  object  of  a  telephone  exchange  is  to  afford  means  for 
placing  any  telephone  user  (subscriber)  into  communication  with 
any  other  subscriber  in  the  same  system.  The  lines  leading  to 
the  telephone  instruments  of  the  various  subscribers  radiate  from 
a  central  point  where  they  terminate  in  an  apparatus  known  as 
a  switch-board. 

Switch-boards  may  be  divided  into  two  classes,  manual  and 
automatic.  In  the  manual  switch-board,  operators — girls — are 
employed  to  make  the  connections  called  for,  while  in  the  au- 
tomatic, the  operation  of  connecting  or  disconnecting  lines  is 
performed  by  the  subscriber  desiring  the  connection.  The 
manual  switch-board  only  will  be  considered  at  present,  as  it  is 
in  almost  universal  use,  the  automatic,  owing  to  its  great  and 
necessary  complexity,  having  proven  successful  only  in  rare  cases. 

The  simplest  form  of  switch-board,  one  typical  of  the  kind 
used  in  small  exchanges  and  designed  for  use  on  grounded  or 
common-return  systems,  will  first  be  considered. 

Each  line  entering  the  exchange  terminates  in  what  is  known 
as  a  spring-jack.  Spring-jacks  are  sockets  containing  or  asso- 
ciated with  simple  switching  devices  and  are  mounted  on  the 
face  of  the  board  within  easy  reach  of  the  operator.  In  order 
to  make  a  connection  with  any  line,  plugs  are  provided  which 
may  be  inserted  into  the  jacks,  and  thus  continue  the  electrical 
path  from  the  line  wire  terminating  therein,  to  and  through  a 
flexible  conducting  cord  attached  to  the  plug. 

Fig.  131  shows  a  simple  spring-jack  with  a  connecting  plug 
inserted.  The  metallic  base,  a,  of  the  jack,  usually  of  brass,  is 
drilled  from  its  forward  end  to  receive  the  shank  of  the  plug,  P. 

A  forwardly  projecting  sleeve  on  this  base  fits  snugly  into  a 
hole  bored  in  the  front  board,  A,  of  the  switch-board,  to  which 
it  is  fastened  by  the  shoulder  and  small  wood-screw  as  shown. 
Firmly  secured  to  the  rear  end  of  the  piece,  a,  is  the  line  spring, 
c,  formed  with  a  rearwardly  projecting  tongue,  to  which  the 
wire,  /,  leading  from  the  line  is  soldered.  The  forward  end  of 
the  spring,  c,  rests  normally  against  the  pin,  /,  carried  by,  but  in- 

153 


154  AMERICAN   TELEPHONE  PRACTICE. 

sulated  from,  the  base,  a.  A  wire,  g,  leads  from  this  pin,  and 
through  the  coil  of  the  line  annunciator  or  drop  to  ground. 
When  the  plug  is  inserted  in  the  jack  its  conducting  tip  makes 
contact  with  the  tip  of  the  line  spring  and  at  the  same  time 
forces  it  out  of  engagement  with  the  pin,  /.  Normally,  therefore, 


Fig.  131. — Spring-Jack. 

the  line  wire  is  connected  to  the  ground  through  the  wire,  /, 
spring,  c,  pin,/,  wire,  g,  and  line-drop  to  the  ground  connection. 
When  the  plug  is  inserted  in  the  jack,  however,  the  line  is  dis- 
connected from  the  branch  leading  through  the  drop,  but  is  con- 
nected through  the  medium  of  the  plug  to  the  flexible  cord. 

Fig.    132  shows  a  common  form  of  switch-board  drop.     The 
purpose   of  the  drop  is  to  attract  the  attention  of  the  operator 
whenever  any  subscriber  wishes  a  connection. 
The  coil  of  the  electromagnet  is  mounted  on 
the  back  of  the  front  plate,  c,  of  the  switch- 
board, as  shown.     To  the  armature,  a,  pivoted 
at  its  upper  end,  is  attached  a  rod,  #,  passing 
forward  through  a  hole  in  the  front  plate  and 
provided    with    a   hook    on    its    forward    end, 
Fig.  132.— Switch-     adapted    to    engage    the    upper   portion    of   a 
Board  Drop.          pivoted  drop-shutter,  s,  and  to  hold  it  in  its 
raised  position.     The  attraction  of  the   arma- 
ture due  to  a  current  passing  through  the  coil  causes  the  hook 
to    rise,  thus   releasing  the  shutter,  which  falls  to   a  horizontal 
position  and  displays  to  the  operator  the  number  by  which  that 
line  is  designated. 

In  order  to  attract  the  attention  of  the  operator  at  night  or 
at  such  times  as  she  may  not  be  in  sight  of  the  board,  a  night- 
alarm  attachment  is  provided  on  each  drop,  which  serves  to  close 
the  circuit  through  a  battery  and  vibrating  bell  whenever  the 
shutter  is  down.  The  small  cam  surface  on  the  lower  portion  of 
the  shutter,  s,  forces  the  light  spring,  /,  into  contact  with  the 


SIMPLE   SWITCH-BOARDS  FOR   SMALL   EXCHANGES.         155 

pin,  /',  when  the  shutter  is  down,  thus  accomplishing  the  above 
result. 

Fig.  133  shows  diagrammatically  the  circuits  of  such  a  switch- 
board.    But   two  subscribers'    lines  with   their  spring-jacks  and 


Line.  Jacls 


Line  JJrops. 


Fig-  133- — Switch-Board  for  Grounded  Lines. 

drops  are  shown.  These,  it  will  be  noted,  enter  by  the  line 
spring  of  the  jack,  and  thence  when  the  plug  is  not  inserted  their 
circuits  pass  through  the  contact  pin  of  the  jack  through  the 
electromagnets  of  their  respective  drops  and  to  ground  at  G. 
In  the  lower  portion  of  the  figure,  R  represents  the  operator's 
receiver,  T  her  transmitter,  B  the  transmitter  battery,  s  and  p 
respectively  the  secondary  and  primary  windings  of  the  opera- 
tor's induction  coil,  Pand  P  a  pair  of  plugs,  and  K,  K'y  and  K" 
keys  connected  therewith,  the  purpose  of  which  will  be  described 
later.  When  one  of  the  line  drops  falls,  indicating  that  the 
subscriber  ^n  thp-f  .!«"«•*  desires  a  connection,  the  operator  takes 


156  AMERICAN    TELEPHONE  PRACTICE. 

up  the  plug,  P ,  and  inserts  it  into  the  jack  bearing  the  cor- 
responding number,  say,  No.  20.  She  then  moves  the  lever  of 
the  key,  K" ,  into  the  position  shown — that  is,  so  that  the  spring 
of  this  key  makes  contact  with  the  stop  below.  This  movement 
connects  the  operator's  telephone  set  with  the  telephone  of  the 
subscriber  calling,  the  circuit  being  traced  from  ground  at  the 
subscriber's  station  through  his  instrument  to  his  line  wire,  from 
the  line  wire  to  the  line  spring  in  the  jack,  thence  to  the  plug, 
P',  cord,  c',  to  the  lever  of  the  key,  K',  through  the  upper  contact 
of  this  lever  to  the  lever  of  key,  K' ',  thence  through  the 
operator's  receiver  and  the  secondary  of  her  induction  coil  to 
ground,  G. 

She  now  ascertains  from  the  subscriber  the  number  of  the  line 
with  which  he  desires  connection,  which  we  will  say  is  No.  63. 
She  thereupon  takes  up  the  other  plug,  P,  of  the  pair  and  inserts 
it  into  jack  63.  In  order  to  call  subscriber  No.  63,  she  presses 
the  key,  K,  into  contact  with  its  lower  stop.  This  completes 
connection  from  the  ground  at  the  central  office,  through  the 
operator's  generator,  through  key,  K,  cord,  c,  plug,  P,  jack  No.  63 
to  subscriber  No.  63,  and  through  the  ringer  magnet  of  his  in- 
strument to  ground.  All  keys  being  in  their  raised  position,  the 
two  subscribers  may  converse  with  each  other  over  the  following 
path:  line  wire  No.  20  to  jack  No.  20,  plug,  P,  cord,  c ,  key,  K'y 
through  the  upper  contact  of  this  key,  through  the  coil  of  the 
clearing-out  drop  to  key,  K,  thence  through  cord,  c,  plug,  P,  jack 
63  to  subscriber  63.  In  case  at  any  time  the  operator  wishes  to 
"  listen  in  "  to  ascertain  if  the  parties  are  through  talking,  she 
may  do  so  by  depressing  key,  K",  which  throws  her  telephone 
into  a  branch  or  derived  circuit  of  the  circuit  between  the  two 
subscribers.  The  key,  K',  may  be  used  to  connect  the  generator 
with  the  line  to  which  the  plug,  P',  is  connected. 

The  clearing-out  drop  is  placed  in  the  circuit  between  the  two 
plugs  to  indicate  to  the  operator  when  either  of  the  subscribers 
turns  his  generator  to  ring  off. 

But  a  single  pair  of  plugs  with  their  corresponding  keys  and 
clearing-out  drop  are  shown,  for  simplicity's  sake.  It  is  usual  to 
place  ten  of  such  pairs  of  plugs  for  each  one  hundred  sub- 
scribers in  the  system,  it  being  found  that  this  number  is 
sufficient  to  meet  the  requirements  at  the  busiest  periods  of 
the  day. 

The  drops  in  a  board  of  this  type  are  usually  wound  to  a  resist- 
ance of  about  80  ohms,  unless  designed  for  multiple  or  bridged 
telephone  lines,  in  \^hich  case  the  resistance  of  the  drops  is  the 


SIMPLE   SWITCH-BOARDS  FOR   SMALL   EXCHANGES.         157 

same  as  that  of  the  ringer  coils  of  the  telephone  instruments  on 
that  line,  usually  1000  ohms. 

This  switch-board  has  not  been  described  because  it  is  a  fail- 
example  of  modern,  up-to-date  apparatus,  but  because,  stripped 
of  all  complicated  devices  for  facilitating  the  work  of  the  opera- 
tor, it  can  be  more  easily  comprehended  by  the  beginner.  A 
large  number  of  such  switch-boards  are,  however,  in  use,  and  for 
small  exchanges  may  give  as  good  service  as  it  is  possible  to 
obtain  with  grounded  or  common-return  lines. 

It  has  already  been  pointed  out  that  in  order  to  avoid  induc- 
tion and  other  sources  of  trouble,  metallic  circuits  are  rapidly 
superseding  ground  circuits  in  telephone  exchanges.  The  switch- 
boards in  common  use  for  small  metallic-circuit  exchanges  are 
built  on  the  same  general  principles  as  those  for  grounded 
circuits  just  described,  differing  from  them  only  in  such  details 
as  to  render  possible  the  connections  of  the  two  branches 
of  one  line  with  those  of  another  line  through  the  cord  cir- 
cuits. For  this  purpose  two  separate  contacts  are  provided  in 
each  jack  forming  the  terminals  of  the  two  branches  of  the  line. 
The  plugs  also  have  two  separate  contact-pieces  adapted  to 
register  with  the  contact-pieces  in  the  jack  when  a  connection  is 
made.  Each  contact  on  the  plug  is  connected  to  a  similar  con- 
tact on  the  other  plug  of  a  pair  through  the  medium  of  a  double- 
conductor  flexible  cord. 

One  form  of  metallic-circuit  jack  is  shown  in  Fig.  134.  Here 
the  tubular  portion,  a  b,  forms  a  terminal  for  one  side  of  the  line, 


Fig.  134.— Metallic-Circuit  Jack. 

while  the  flexible  spring,  d,  forms  the  terminal  for  the  other  side 
of  the  line.  The  terminal,^,  connected  with  the  pin  upon  which 
the  spring,  d,  normally  rests,  forms  one  terminal  for  the  coil  of 
the  line-drop.  The  other  terminal  of  this  coil  is  attached  to  the 
terminal,  a,  so  that  when  the  spring,  d,  is  in  contact  with  its  pin 
the  circuit  is  complete  from  one  side  of  the  line  to  the  other 
through  the  drop  coil.  The  tubular  frame  of  this  jack  is  made  in 
two  pieces,  a  and  b.  The  front  portion,  b,  is  a  hollow  screw, 


158 


AMERICAN    TELEPHONE   PRACTICE. 


threaded  to  engage  a  tapped  hole  in  the  front  of  the  piece,  a. 
By  this  arrangement  any  jack  may  be  readily  removed  from  the 
board  by  unscrewing  the  piece,  b,  until  it  disengages  the  rear 
portion,  a.  A  slot  for  receiving  a  screw-driver  is  provided  on  the 
front  of  the  piece,  b,  to  accomplish  this. 

A  metallic-circuit  plug  in  common  use  is  shown  in  Fig.  135. 
The  tip  conductor  is  formed  of  a  rod  of  brass  slightly  enlarged  at 


t    b 


Fig.  135.— Metallic-Circuit  Plug. 

its  forward  end.  This  is  encased  in  a  bushing,  b,  of  hard  rubber, 
and  over  this  is  slid  a  tube,  s,  of  brass  forming  the  sleeve  of  the 
plug.  A  second  bushing,  #',  covers  the  rear  portion  of  the 
sleeve,  j,  and  the  rear  portion  of  this  latter  tube  is  in  turn  cov- 
ered by  the  tube,  b",  of  hard  rubber,  forming  the  handle  of  the 
plug.  The  tube,  s,  forming  the  sleeve  has  a  portion  which  pro- 
jects rearwardly  into  the  handle  and  is  there  provided  with  a 
connector,  £,  to  which  the  terminal  of  one  conductor  of  the  flex- 
ible cord  is  attached.  The  other  connector,  c',  is  attached  to 
the  rear  portion  of  the  tip  piece,  t,  and  forms  the  terminal  for 
the  other  conductor  of  the  cord. 

In  Fig.  136  is  shown  a  form  of  jack  and  plug  manufactured 
by   the    American    Electric   Telephone    Co.     The   jack    is   self- 


Fig.  136.— American  Jack  and  Plug. 


Fig.  137. — Keystone  Telephone 
Co.'s  Jack  and  Plug. 


contained  and  is  mounted  on  the  board  by  means  of  a  screw- 
threaded  thimble,  in  much  the  same  manner  as  the  jack  shown 
in  Fig.  134.  The  two  springs  are  secured  rigidly  to  the  frame  of 
the  jack,  but  are  insulated  from  it  and  from  each  other  by  strips 
of  hard  rubber  and  by  insulating  bushings  for  the  screws.  The 
plug  differs  in  its  details  from  that  shown  in  Fig.  135,  but  its 


SIMPLE   SWITCH-BOARDS  FOR   SMALL   EXCHANGES. 


'59 


contacts  perform  the  same  functions.  The  entire  metal  portion 
of  the  plug,  including  the  tip  and  sleeve  contacts,  are  screw- 
threaded  into  the  hard-rubber  bushing,  forming  the  handle,  and 
make  contact  with  the  terminals  of  the  flexible  cord  in  such 
manner  as  to  bind  it  firmly  in  place  without  the  use  of  other 
connectors.  The  screw-threaded  thimble  of  the  jack  is  provided 
with  a  long  shank  so  as  to  adapt  it  to  fit  almost  any  thickness  of 
panel  board.  This  jack  and  plug -are  made  with  special  reference 
to  use  upon  boards  already  installed,  when  it  is  desired  to  in- 
crease their  capacity. 

In  Fig.  137  is  shown  another  form  of  jack  and  plug,  manu- 
factured by  the  Keystone  Telephone  Co.  of  Pittsburg,  Pa.  The 
construction  and  operation  of  this  are  evident  from  the  cut. 

In  Fig.  138  is  shown  in  diagrammatic  form  the  circuits  of  a 
switch-board  of  this  class.  Here  the  line  wires,  /J  and  /2,  forming 


C.O. 


XT* 


iJSD 


Fig.  138.— Circuits  of  Metallic  Switch-Board. 

the  two  sides  of  a  metallic  circuit,  enter  the  spring-jacks,  e,  e1,  and 
e'\  in  the  manner  described  in  connection  with  Fig.  133.  It  will 
be  noticed  that  while  the  tip-spring,  d,  is  in  its  normal  position, 
circuit  is  traced  from  the  line,  /',  through  the  coil  of  the  drop,/, 
and  back  to  line,  /2,  so  that  current  sent  from  the  subscriber's 
station  will  actuate  the  drop,  thus  indicating  a  call.  When  one 
of  the  plugs,  P  or  P',  is  inserted  into  the  jack  spring,  d  is  raised 
from  its  normal  resting-place  and  breaks  contact  with  the  termi- 
nal leading  to  the  drop-coil,  thus  cutting  the  drop  out  of  the 
circuit.  At  the  same  time,  the  connection  is  continued  from  the 
two  line  wires,  /'  and  /2,  to  the  two  strands  of  the  cord  circuit. 
When  an  operator  notices  that  a  drop  has  fallen  she  inserts  the 
answering  plug,  P,  into  the  jack  corresponding  to  that  drop  and 
by  pressing  the  button,  K,  belonging  to  that  cord  circuit,  bridges 
her  telephone  set,  7",  across  the  two  strands,  I  and  2,  of  the  cord 
circuit.  This  enables  her  to  communicate  with  the  subscriber  call- 


160  AMERICAN    TELEPHONE  PRACTICE. 

ing,  to  ascertain  his  wants.  She  then  inserts  the  calling  plug,  P, 
into  the  jack  of  the  called  subscriber  and  presses  the  button,  K  , 
thus  connecting  the  terminal  of  the  generator,  G,  with  the  two 
sides  of  the  line  of  the  subscriber  called. 

It  will  be  noticed  that  when  the  key,  K  ,  is  in  its  normal  posi- 
tion the  conductors  from  the  tip  and  sleeve  of  the  answering  plug 
to  the  tip  and  sleeve  of  the  calling  plug  are  made  continuous  by 
the  springs  of  the  calling  key  resting  against  their  inside  anvils. 
When  the  key  is  depressed  the  springs  break  contact  with  the 
inside  anvils,  thus  severing  the  connection  between  plugs,  P 
and  P',  and  immediately  afterward  connect  with  the  outside 
anvils  forming  the  terminals  of  the  generator,  G,  thus  sending 
current  over  the  called  subscriber's  line. 

The  clearing-out  drop,  C  O,  is  permanently  bridged  across  the 
cord  circuit  as  shown,  in  order  to  indicate  to  the  operator  when 
either  subscriber  rings  off.  In  order  that  the  efficiency  in  talk- 
ing may  not  be  impaired,  this  drop  is  made  of  high  resistance 
and  high  impedance. 

The  line-drops  are  usually  of  the  ordinary  type  described  in 
connection  with  the  grounded-circuit  switch-board.  The  clear- 
ing-out drops,  however,  must  be  made  to  meet  more  difficult 
requirements  than  the  line-drops.  As  they  are  always  bridged 
across  the  circuit  of  two  connected  subscribers,  it  is  found  that 
unless  special  precautions  are  taken  much  trouble  will  be  expe- 
rienced from  cross-talk  due  to  induction  between  two  adjacent 
drops.  This  difficulty  cannot  be  overcome,  as  in  the  line-drops, 
by  cutting  them  out  of  the  circuit  whenever  two  subscribers  are 
connected,  inasmuch  as  the  very  purpose  for  which  they  exist 
requires  them  to  be  always  in  such  circuits.  Neither  can  it  be 
overcome  by  placing  the  drops  at  such  a  distance  from  one 
another  that  this  induction  will  not  be  felt,  for  the  limited  space 
on  switch-boards  requires  that  they  be  put  as  close  together  as 
mechanical  conditions  will  allow. 

It  has  thus  been  found  necessary  to  design  drops  which  would 
neither  affect  nor  be  affected  by  any  similar  drop  in  its  imme- 
diate vicinity.  This  has  been  accomplished  in  several  ways,  but 
the  best  example  is  that  shown  in  Fig.  139,  which  illustrates 
what  is  known  as  the  "  Warner  Drop."  In  this  the  coil  is  wound 
in  the  ordinary  manner  on  a  soft-iron  core  and  is  then  encased  in 
a  tubular  shield,  c,  also  of  soft  iron.  The  armature,  d,  is  pivoted 
at  points,  e,  in  a  bracket,/,  mounted  directly  on  the  rear  portion 
of  the  tubular  magnet.  From  this  armature  a  rod,  #,  extends 
forward  through  a  notch  in  the  front  plate,  b,  in  such  manner  as 


SIMPLE   SWITCH-BOARDS  FOR   SMALL   EXCHANGES.         l6l 

to  engage  the  upper  portion  of  the  shutter  and  thus  hold  it  in  its 
raised  position.  A  screw,  /,  passing  through  the  front  plate,  />, 
serves  not  only  to  hold  the  magnet  in  place,  but  to  hold  the 
core  in  its  place  within  the  shell.  The  terminals  of  the  coil  are 
led  out  through  two  small  holes  in  the  armature  and  are  con- 
nected with  the  terminals,//  Amounted  on  an 'insulating  strip, 
carried  on  the  bracket,/. 

These  drops  should  be  so  nicely  made  that  the  armature,  d, 
will  fit  closely  against  the  end  of  the  tube,  c,  in  such  manner  as 


Fig.  139. — The  Warner  Drop. 

to  almost  completely  close  the  magnetic  Circuit  in  which  the  coil 
is  placed.  The  lines  of  force  generated  by  the  passage  of  a  cur- 
rent through  the  coil  follow  almost  entirely  the  path  provided  for 
them  by  the  shell  and  the  core  of  the  magnet,  thus  not  only  pro- 
ducing a  very  efficient  electromagnet,  but  also  preventing  any 
of  the  lines  of  force  from  extending  beyond  the  limits  of  the 
shell.  These  drops  are  usually  wound  to  a  resistance  of  500 
ohms  and  may  be  mounted  as  closely  together  as  desired  with- 
out producing  perceptible  cross-talk.  The  impedance  due  to  the 
great  number  of  turns  in  the  coil,  and  to  the  perfect  magnetic 
circuit  surrounding  the  same,  is  so  great  that  practically  no 
diminution  in  the  strength  of  speech  transmission  is  felt  due  to 
its  being  bridged  across  the  two  sides  of  the  line. 


162 


AMERICAN    TELEPHONE  PRACTICE. 


Another  form  of  tubular  drop  is  shown  in  Fig.  140.  This  is 
manufactured  by  the  American  Electric  Telephone  Co.,  and 
needs  but  slight  description.  The  tubular  magnet  is  mounted 
on  a  brass  bracket  extending  from  the  rear  plate  of  the  switch- 
board, upon  the  front  of  which  is  pivoted  the  shutter.  The 
armature  is  pivoted  at  its  lower  edge  in  the  brass  bracket,  and 
carries  on  its  upper  side  a  forwardly  projecting  rod  which  serves 
as  a  catch  for  the  shutter.  This  drop  gives  excellent  service  in 


Figs.  140  and  141. — American  and  Keystone  Tubular  Drops. 

practice,  but  is  probably  not  quite  so  sensitive  as  the  Warner 
drop,  because  the  armature  in  its  backward  movement  must 
necessarily  pull  the  shutter  back  slightly  before  it  can  release  it. 
This  is  but  a  slight  objection,  however,  and  does  not,  as  stated 
above,  seriously  impair  its  efficiency. 

In  Fig.  141  is  shown  the  tubular  drop  of  the  Keystone  Tele- 
phone Company,  the  arrangement  of  the  parts  being  evident 
from  the  cut. 


CHAPTER    XV. 

LISTENING  AND    RINGING    APPARATUS   FOR   SWITCH-BOARDS. 

IN  order  to  accomplish  the  changes  of  circuit  by  which  the 
operator  is  enabled  to  connect  her  telephone  with  the  line  of  any 
subscriber,  and  to  send  calling  current  to  actuate  the  bells  at  any 
subscriber's  station,  many  forms  of  circuit-changing  switches 
have  been  devised.  One  of  these,  shown  in  Fig.  142  and  known 


h- 


Fig.  142.  —  O'Connell  Key. 


as  the  O'Connell  key,  has  been  in  use  in  this  country,  either  in 
the  form  shown  or  with  slight  modifications,  for  many  years. 
Six  different  contact-springs  are  so  mounted  and  formed  as  to  be 
acted  upon  by  a  wedge,  b,  of  insulating  material  adapted  to  slide 
vertically  among  them.  This  wedge  is  mounted  upon  a  rod, 
carrying  at  its  upper  end  a  button,  by  which  it  may  be  raised  or 
lowered  by  the  operator.  The  two  springs,  £3,  form  the  terminals 
of  the  two  strands  of  the  flexible  cord  leading  respectively  to  the 
tip  and  sleeve  of  the  calling  plug.  These  springs  are  provided 
with  rollers,  b\  in  order  to  reduce  friction  when  acted  upon  by 
the  wedge,  b.  The  springs,  b\  form  respectively  the  terminals 
for  the  strands  of  the  cord  leading  to  the  tip  and  sleeve  of  the 
answering  plug.  The  two  springs,  bz,  are  connected  each  to  one 
terminal  of  the  operator's  set,  while  the  pins,  b'\  are  connected 
each  to  one  terminal  of  the  calling  generator.  The  normal  posi- 
tion of  this  apparatus  is  when  the  wedge  is  raised  to  its  highest 
position.  In  this  position  the  springs,  #*,  rest  against  the  smallest 
portion,  b\  of  the  wedge,  $,  and  are  not  in  engagement  with  the 
springs,  #3.  The  springs,  tf,  however,  rest  against  the  springs,  b\ 
thus  making  complete  the  connection  from  the  tip  and  sleeve  of 
the  answering  plug  to  the  tip  and  sleeve,  respectively,  of  the  call- 

163 


i64 


AMERICAN   TELEPHONE  PRACTICE. 


ing  plug.     In  this  position  two  subscribers  may  converse  without 
being  heard  by  the  operator. 

When  the  button  is  depressed  one  notch  the  springs,  £2,  ride 
upon  the  second  portion  of  the  wedge,  b,  thus  forcing  them  into 
engagement  with  the  springs,  £3,  without  causing  these  latter  to 
break  contact  with  the  springs,  b\  In  this  position  the  circuit 
between  the  two  plugs  is  not  broken,  but  the  operator's  tele- 
phone set  is  connected  across  the  two  strands  of  the  cord,  thus 
allowing  the  operator  to  listen  in  and  to  communicate  with  either 
of  the  two  subscribers  who  are  talking.  In  its  third  position, 
which  is  that  shown  in  Fig.  143,  the  springs,  £3,  break  contact 


Figs.  143  and  144. — O'Connell  Key. 

with  both  springs,  b1  and  b\  and  come  into  contact  with  the 
pins,  b\  which  are  connected  with  the  terminals  of  the  generator. 
This  sends  calling  current  to  the  called  subscriber  without  affect- 
ing in  any  manner  the  circuits  leading  to  the  calling  subscriber. 
When  the  button  is  depressed  to  its  utmost  extent  the  springs, 
//,  are  pressed  outwardly  as  is  shown  in  Fig.  144,  until  they  not 
only  make  contact  with  pins,  b\  but  also  with  pins,  b\  These 
pins,  b\  are  each  connected  to  pins,  £8,  against  which  the  springs, 
b\  are  now  resting.  This  completes  a  circuit  from  the  generator 
terminal,  £5,  through  the  springs,  b\  to  the  pins,  //,  thence  to  the 
pins,  b*,  and  thence  through  the  springs,  b\  to  the  sleeve  and  tip 
of  the  answering  plug  and  to  the  line  of  the  calling  subscriber. 
Thus,  in  this  final  position  of  the  key,  calling  current  is  sent  not 
only  to  the  subscriber  to  be  called,  but  also  to  the  one  who 
originated  the  call.  Of  course,  this  is  necessary  only  when  for 
some  reason  the  calling  subscriber  has  left  his  instrument. 


LISTENING  AND   RINGING  APPARATUS. 


165 


In  later  and  better  keys,  arrangement  is  made  whereby  either 
the  calling  or  the  called  subscriber  may  be  called  without  dis- 
turbing the  other. 

The  combined  listening  and  ringing  key  shown  in  Fig.  145 
is  the  invention  of  Mr.  Frank  B.  Cook,  of  the  Sterling  Electric 


•*? 


Fig.  145. — Cook  Key. 

Co.  This  key  is  quite  extensively  used  by  some  of  the  licensees 
of  the  Bell  Company,  and  also  in  all  of  the  switch-boards  manu- 
factured by  the  Sterling  Company.  The  springs  in  this  key 
are  arranged  in  duplicate  sets,  the  two  sets  being  divided  by  a 
hard  rubber  partition,  18,  as  shown  in  the  sectional  view  at  the 
bottom  of  the  figure.  The  two  sets  of  springs  are  shown  sepa- 
rated in  the  upper  portion  of  the  figure,  and  their  various  circuit 
connections  clearly  indicated.  The  springs  are  acted  upon  by 
the  cam,  12,  of  hard  rubber  pivoted  in  the  metal  frame,  and  adapted 


i66 


AMERICAN   TELEPHONE  PRACTICE. 


to  be  turned  through  a  small  arc  by  the  handle,  15.  The  springs 
23,  on  each  side  of  the  partition  bear  against  the  left-hand  portion 
of  the  cam,  and  form  the  terminals  of  the  operator's  talking 
circuit  including  her  receiver  and  the  secondary  of  her  induction 
coil.  On  the  opposite  side  of  the  cam  are  the  two  springs,  24, 


Fig.  146. — American  Key. 

forming  the  terminals  of  the  clearing-out  drop,  E,  one  of  them 
being  placed  on  each  side  of  the  partition.  The  springs,  28, 
form  the  terminals  of  the  tip  and  sleeve  strands  of  the  calling 
plug,  while  the  springs,  29,  form  the  terminals  of  the  two  corre- 
sponding strands  of  the  answering  plug.  These  springs  normally 
rest  against  the  two  contact  strips,  36,  so  that  the  tip  of  the  calling 
plug  is  normally  connected  through  one  of  the  strips,  36,  to  the 
tip  of  the  answering  plug,  and  similarly  the  sleeve  of  the  calling 
plug  with  the  sleeve  of  the  answering  plug  through  the  other 
strip,  36.  When  the  cam  is  rotated  in  one  direction  the  listening 


LISTENING  AND   RINGING  APPARATUS.  167 

springs,  23,  are  pressed  into  engagement  with  the  two  strips,  36, 
thus  bridging  the  operator's  telephone  across  the  two  sides  of  the 
cord  circuit.  When  the  cam  is  in  its  opposite  position  the  two 
clearing-out  springs,  24,  are  pressed  into  engagement  with  these 
strips,  thus  bridging  the  clearing-out  drop  across  the  cord  circuit. 
This  latter  position  is  the  normal  position  of  the  cam.  Two 
outside  terminal  strips,  37,  are  provided,  one  on  each  side  of  the 
partition,  these  forming  the  terminals  of  the  switchboard 
generator,/.  By  means  of  pressure  on  one  of  the  buttons,  16 
or  17,  the  contact  springs,  29  or  28,  may  be  pressed  out  of 
engagement  with  the  strips,  36,  and  into  engagement  with 
the  generator  terminals,  37,  thus  disconnecting  the  strands  of 
the  cord  from  the  rest  of  the  circuit  and  at  the  p~i 
same  time  connecting  them  with  the  genera- 
tor terminals.  Upon  releasing  the  button  the  \ 

springs  resume  their  normal  position,  completing 
the  circuit  between    the    two    plugs  and  cutting 
out  the  generator.     These  circuit  changers  have 
the    advantage   of    being  entirely  self-contained, 
thus   rendering  the   removal   of  any  one  of  them 
from  the  switch-board  a  comparatively  easy  matter 
when  repairs  are  necessary.     They  are  entirely  in- 
closed, and  are  therefore  quite  free  from  dust,  which  Fig.  147.— Section- 
causes  much  trouble  in  the  way  of  poor  connec-    al  View  Ameri- 
tions  in  many  otherwise  efficient  circuit  changers. 

A  new  key,  just  put  upon  the  market  by  the  American  Electric 
Telephone  Co.,  is  shown  complete  in  Fig.  146  and  in  sec- 
tion in  Fig.  147.  In  this  all  of  the  listening  and  ringing  opera- 
tions are  performed  by  the  manipulating  of  a  single  lever,  no 
buttons  being  required  to  p-erform  the  ringing,  as  is  usually  the 
case.  As  in  the  Cook  key,  two  sets  of  springs  are  provided, 
being  separated  by  a  partition,  A,  of  hard  rubber.  The  springs 
are  mounted  in  slits  cut  in  hard-rubber  blocks,  B,  these  blocks 
being  clamped  between  two  brass  side-plates  forming  the  frame 
of  the  circuit  changer.  The  front  one  of  these  side-plates  is 
removed  in  Fig.  147  in  order  to  better  show  the  construction. 

The  circuits  of  this  apparatus  are  shown  in  Fig.  148,  the  two 
sets  of  springs  being  separated  in  order  to  render  their  action 
clearer.  It  should  be  remembered,  however,  that  the  cam,  C,  acts 
in  the  same  manner  on  each  set  of  springs,  so  that  the  two  sets 
always  occupy  similar  positions.  The  pair  of  springs,  I,  form  the 
terminals  of  the  operator's  set,  and  are  adapted  to  make  contact 
with  the  springs,  2,  when  the  lever  is  pressed  to  the  right.  The 


i68 


AMERICAN   TELEPHONE  PRACTICE. 


springs,  2,  normally  bear  against  the  springs,  4,  which  form  the 
terminals  of  the  tip  and  sleeve  strands  respectively  of  the  answer- 
ing plug,  P.  The  springs,  3,  make  normal  contact  with  the  springs, 
5,  which  form  the  terminals  of  the  tip  and  sleeve  strands  of  the 
calling  plug,  P' .  As  the  springs,  2  and  3,  on  each  side  are  perma- 
nently connected  together,  it  follows  that  in  the  normal  position 
of  the  circuit  changer  the  tip  strand  is  made  complete  through 
spring,  4,  spring,  2,  spring,  3,  and  spring,  5,  and  the  sleeve  strand 


Fig.  148. — Circuits  American  Key. 

is  made  complete  through  the  same  springs  on  the  other  side  of 
the  partition.  When  the  cam  lever  is  thrown  to  the  right,  the 
springs,  I,  are,  as  before  stated,  pressed  into  engagement  with 
the  springs,  2,  thus  bridging  the  operator's  telephone  across  the 
combined  cord  circuit.  When  the  lever  is  pressed  still  further 
to  the  right,  the  rubber  plate,  D,  carried  upon  it,  presses  the 
springs,  5,  into  engagement  with  the  springs,  6,  at  the  same  time 
breaking  the  contact  with  the  springs,  3.  As  the  springs,  6,  form 
the  terminals  of  the  switch-board  generator,  this  sends  calling 
current  over  the  line  with  which  the  calling  plug,  P',  is  connected. 
No  current  is  sent  to  the  line  with  which  the  other  plug  is  con- 
nected, because  the  circuit  is  broken  between  springs,  3  and  5,  on 
each  side  of  the  partition.  In  a  similar  manner  a  pressure  of  the 
lever  to  the  extreme  left  causes  the  springs,  4,  to  break  engage- 


LISTENING  AND  RINGING  APPARATUS. 


169 


ment  with  the  springs,  2,  and  to  come  in  contact  with  the  springs, 
7,  which  also  form  terminals  of  the  calling  generator.  This  sends 
calling  current  over  the  line  with  which  the  plug,/*,  is  connected. 
The  combined  listening  and  ringing  key  of  the  Western 
Telephone  Construction  Co.  is  operated  entirely  by  one  lever, 
this  lever  being  normally  held  in  ( its  central  position  by  the  con- 
tact springs  against  which  it  operates.  In  order  to  connect  the 
operator's  telephone  across  the  terminals  of  the  cord  circuit  with 
this  key,  the  lever  is  pushed  straight  down  without  rocking  it  at 


Fig.  149. — American  Listening  Key. 

all,  this  action  pressing  the  light  operator's  springs  into  contact 
with  the  tip-  and  sleeve-springs  of  the  cord  circuit.  When  the 
lever  is  rocked  toward  the  operator,  the  tip-  and  sleeve-springs 
of  the  calling  plug  are  pressed  into  engagement  with  the  gener- 
ator contacts,  thus  at  the  same  time  cutting  off  the  connection 
with  the  tip-spring  of  the  answering  cord.  A  rocking  motion 
in  the  other  direction  presses  the  tip-  and  sleeve-springs  of  the 
answering  plug  into  engagement  with  the  generator  contacts,  at 
the  same  time  cutting  off  the  tip-springs  of  the  calling  cord. 

Fig.  149  shows  a  simple  key  manufactured  by  the  American 
Electric  Telephone  Co.,  and  typical  of  a  large  number  of  keys 
designed  for  the  purpose  of  either  listening  or  ringing.  In  this 
the  plunger,  A,  is  of  hard  rubber,  and  is  normally  pressed  into 


1 70  AMERICAN   TELEPHONE   PRACTICE. 

its  upper  position  by  the  spiral  spring  below  it.  It  may  be 
depressed,  however,  by  means  of  the  lever,  in  an  obvious 
manner.  The  outside  springs  form  the  terminals  of  the  cord 
circuit,  while  the  inside  shorter  springs  may  form  the  terminals 
of  the  operator's  telephone  set  or  of  the  calling  generator, 
according  as  to  whether  the  key  is  to  be  used  for  listening,  or  for 
ringing  purposes.  When  the  plunger  is  depressed,  the  outside 
springs  fall  into  the  depression  on  the  plunger,  while  the  shorter 
springs  ride  upon  its  enlarged  portion,  thus  pressing  the  two  pairs 
of  springs  into  contact  and  bridging  the  telephone  or  generator 
across  the  cord  circuit. 

To  facilitate  the  manipulation  of  switch-boards  it  is,  of  course, 
desirable  to  make  the  number  of  motions  necessary  to  effect 
a  connection  as  few  as  possible.  If,  therefore,  some  act  which 
must  necessarily  be  performed  by  the  operator  in  inserting  a 
plug  in  or  withdrawing  it  from  a  jack  can  be  made  use  of  to 
bring  about  some  of  the  other  changes  of  circuit,  a  decided 
advantage  is  gained.  In  Fig.  150  a  device  for  accomplishing 


Fig.  150.— Plug  Listening  Device. 

this  is  shown.  The  line  springs  of  a  jack  are  represented  by 
a  and  b;  c  and  d  are  two  springs  arranged  adjacent  to  the  line 
springs  and  forming  terminals  of  the  operator's  telephone 
set.  The  plugs  are  formed  with  alternate  depressions  and 
enlargements,  which  are  so  spaced  that  when  the  plug  is 
partially  inserted  into  a  jack  the  two  line  springs  ride  upon  the 
two  enlargements,  thus  pressing  the  line  springs  into  engage- 
ment with  the  operator's  terminals,  c  d.  This  places  the 
operator  into  communicative  relation  with  subscriber.  After 
the  operator  has  learned  the  connections  desired,  she  inserts 
the  plug  fully  into  the  jack,  thus  allowing  the  two  line 
springs  to  fall  into  the  recesses  of  the  plug.  This  maintains 
the  connection  between  the  line  and  the  conductors  of  the  plug, 
but  breaks  the  connection  between  the  operator's  telephone  and 
the  line.  The  apparatus  is  then  in  the  position  shown  in  Fig. 
150.  If  at  anytime  the  operator  wishes  to  listen  in  without 
breaking  the  connection  between  two  subscribers,  she  may  do 
so  by  partially  withdrawing  the  plug.  If  she  finds  that  they 


LISTENING  AND   RINGING  APPARATUS. 


171 


are  through  talking,  the  movement  is  continued  and  the  plug 
replaced  in  its  seat.  Where  this  device  is  used  an  ordinary 
ringing  key  is  required  to  connect  the  generator  across  the 
terminals  of  the  calling  plug. 

In  Figs.  151   and   152  is  shown  a  device  in  common  use,  de- 
signed   by   Mr.   W.   O.    Meissner,  which  accomplishes   the  con- 


Fig.  151. — Meissner  Ringing  Device. 

nection  of  the  calling  generator  with  the  line  of  the  subscriber 
called  for  by  inserting  the  plug  to  its  utmost  extent  into  the  jack. 
The  illustrations  show  a  jack  and  plug  for  common-return  or 
grounded  lines.  In  Fig.  151  the  plug  is  shown  partially  inserted 
into  the  jack,  in  which  position  the  line  spring,  C,  makes  contact 
at  the  point,  C1,  with  the  conductor,  j52,  of  the  plug,  thus  com- 
pleting the  connection  between  the  line,  C*,  and  the  strand,  B\ 
of  the  cord.  In  this  position  the  circuit  is  continuous  between 
two  connected  subscribers  or  between  the  operator  and  one  sub- 
scriber, as  the  case  may  be.  When  it  is  desired  to  ring  out  on 
the  line,  C*,  the  plug  is  pressed  to  its  fullest  extent  into  the  jack, 


Fig.  152.— Meissner  Ringing  Device. 

as  shown  in  Fig.  152.  In  this  position  the  spring,  C,  rides  upon 
the  insulated  sleeve,  J3l,  of  the  plug,  thus  breaking  connection  be- 
tween the  spring  and  the  contact,  B1,  and  at  the  same  time 
pressing  the  tip  of  the  spring  into  contact  with  the  strip,  C9, 
which  is  connected  by  wire,  C\  to  one  terminal  of  the  generator. 
Current  from  the  generator  thus  flows  to  line  until  the  plug  is 
released,  at  which  time  it  is  forced  outward  by  the  action  of  the 
spring  and  again  resumes  the  position  shown  in  Fig.  151. 
Where  this  device  is  used,  listening  in  by  the  operator  is  accom- 
plished by  an  ordinary  listening  key. 


172 


AMERICAN    TELEPHONE  PRACTICE. 


In  Fig.  153  is  shown  another  device  for  listening  in.     Pis  the 
calling  plug  of  any  pair,  and  is  shown  in  its  normal  socket  on  the 


Fig.  153- — Plug  Socket  Listening  Key. 

key  table.  By  tilting  it  in  its  socket  until  it  assumes  the  posi- 
tion shown  iii  the  dotted  lines,  the  spring,  S,  is  forced  from 
its  normal  position,  and  thus  presses  the  springs,  q  and  r,  into 
engagement  with  terminals,  ql  and  r1.  As  is  shown  by  the 
diagram,  this  act  connects  the  operator's  telephone  set  across 
the  two  strands  of  the  cord  circuit,  TT1. 

The  knob,  Q,  upon  the  spring,  5,  may  be  used  to  connect  the 
operator's  telephone  across  the  cord  circuit,  in  case  it  is  desirable 
to  listen  in  after  the  plug,  P,  has  been  removed  from  its  socket. 
Calling  is  done  by  pressing  the  key,  K.  This  affords  a  very 
rapid  means  for  connecting  the  operator's  telephone  into  circuit 
with  any  line,  for  after  having  inserted  an  answering  plug  into 
the  jack  of  a  calling  subscriber,  she  can,  by  part  of  the  move- 
ment which  withdraws  the  calling  plug,  P,  from  its  socket, 
connect  her  telephone  with  the  calling  subscriber's  line.  A 
continuation  of  this  movement  completes  the  connection  with 
the  called  subscriber,  and  at  the  same  time  cuts  the  operator's 
telephone  out  of  circuit. 


CHAPTER   XVI. 


SELF-RESTORING   SWITCH-BOARD    DROPS. 

IT  is  generally  considered  of  great  advantage  to  have  switch- 
boards so  arranged  that  it  will  be  unnecessary  for  the  operator  to 
manually  restore  the  drops.  The  reason  for  this  is  that  every 
movement  on  the  part  of  the  operator,  in  establishing  a  con- 
nection between  two  subscribers,  requires  a  certain  amount  of 
time,  and  that  in  the  busier  portions  of  the  day  an  opera- 
tor is  worked  almost  to  the  extremity  of  her  endurance,  and 
therefore  that  the  saving  of  any  movements  in  handling 
these  connections  will  be  a  great  gain  in  the  rapidity  with  which 
the  board  can  be  operated.  Such  saving  of  the  work  of  the 
operator  not  only  insures  a  quicker  and  therefore  a  better  service, 


s~ 

o 

-  \ 

/•  o 

a 

iz 

Fig.  154. — Self-Restoring  Drop. 

but  also  may  reduce  the  cost  of  the  operation  of  the  exchange 
by  enabling  fewer  operators  to  handle  the  system.  There  are, 
however,  many  who  contend  that  the  greater  part  of  an  operator's 
time  is  necessarily  taken  up  in  talking  or  listening  to  the  sub- 
scriber in  order  to  ascertain  his  wishes,  and  that  while  she  is  doing 
this  she  may  restore  the  drops  by  hand  without  loss  of  time. 
Notwithstanding  this,  however,  the  number  of  exchanges  using 
self-restoring  drops  is  rapidly  increasing,  and  many  inventions 
have  recently  been  made  and  put  into  practice  to  bring  about 
this  result. 

Brief  mention  has  already  been  made  of  the  electrically  re- 
storing switch-board  drops  used  to  a  large  extent  by  the  American 
Bell  Telephone  Company.  The  details  of  such  a  drop  are  shown 
in  Figs.  154,  1 5 5,  and  156.  In  Fig.  154,  a  is  a  tubular  electromag- 


174 


AMERICAN    TELEPHONE   PRACTICE. 


x^f 


Fig.   155.— Self-Re- 
storing Drop. — Sec- 
tional View. 


net,  carrying  on  its  rear  end  an  armature,  C,  pivoted  at  c,  which 
armature  carries  an  arm,  c\  which  projects  forward  and  is  pro- 
vided with  a  catch,  c\  on  its  extremity.  So  far  the  arrangement 
is  almost  identical  with  that  of  the  Warner  tubular  drop  already 
described.  A  second  tubular  electromagnet, 
d,  is  secured  to  the  front  of  the  plate,  b,  which 
also  supports  the  magnet,  a.  This  second 
magnet  has  its  poles  facing  the  front  of  the 
board.  An  armature,  <?,  is  pivoted  at  its 
lower  side  by  the  pivots,  el  and  e*,  shown  in 
Fig.  1 56.  The  catch,  c\  on  the  rod,  c\  is  adapted 
to  engage  a  lug,  e\  on  the  armature  and  retain 
it  in  its  vertical  position.  Pivoted  on  the 
bracket,/,  which  is  insulated  from  the  magnet 
by  the  strip  of  insulating  material,  /',  is  a  light  shutter,  g.  The 
tendency  of  the  armature,^,  when  released  is  to  fall  outward, and 
in  so  doing  it  presses  against  the  light  shutter,  £-,  just  below  its 
pivotal  point,  and  forces  it  into  a  horizontal  position. 

The  coil  of  the  electromagnet^  a,  is  usually  termed  the  line 
coil,  and  is  included  in  the  circuit  of  the  line  wire.  The  coil 
of  the  electromagnet,  d,  termed  the  restoring  coil,  is  in  a  local 
circuit  containing  a  battery  which  is  closed  by  the  insertion-  of  a 
plug  into  the  spring-jack  of  the  line  belonging  to  that  drop. 

Various  arrangements  associating  drops  of  this  type  with  the 
line  circuits  and  with  the  local  circuits  at  the  exchange  have  been 
devised  and  put  into  practical  operation 
with  almost  unqualified  success.  The  ar- 
rangement  in  Fig.  157  is  typical,  and  at  the 
same  time  shows  a  very  interesting  improve- 
ment designed  for  saving  battery  power  in 
the  exchange.  The  ordinary  arrangement 
of  subscriber's  circuit  is  shown,  and  it 


Fig.     156.— Self-Restor- 
be  noted  that  the  actuating  coil,  a,  is  bridged       in_  Dr     —Front 

across  the  two  sides  of  the  line  wire.     The  View, 

coil,  a,   of   course,  is    necessarily  wound  to 

about  500  ohms  resistance  to  prevent  short-circuiting  the  voice  cur- 
rent. Two  sleeves  or  thimbles,  k  k\  are  shown  on  each  jack  of  the 
line,  the  inner  ones,  k,  of  which  are  shown  connected  permanently 
together  and  grounded  through  a  battery,  k*.  The  outer  thimbles, 
k1,  are  connected  together,  and  are  usually  connected  to  the 
ground  directly  through  the  restoring  coil,  d.  When  with  this  ar- 
rangement a  plug  is  inserted,  the  two  thimbles  of  the  jack,  k  k\  are 
short-circuited  by  the  sleeve  on  the  plug,  and  the  circuit  through 


SELF-RESTORING   SWITCH-BOARD  DROPS. 


the  actuating  coil  is  thus  closed  through  the  battery,  k*.  This  pulls 
the  armature,  e,  back  until  it  engages  the  catch,  c\  and  thus  allows 
the  shutter  to  swing  into  its  normal  position.  Any  subsequent 
currents  coming  over  the  line  wire  will  fail  to  operate  the  drop, 
for  the  coil,  d,  will  not  allow  its  armature,  e,  to  fall  against  the 
shutter,  g,  while  the  plug  is  in  the  jack  of  that  line.  This  ar- 


rangement has  been  found  to  be  sub- 
ject to  one  somewhat  serious  objec- 
tion, viz.,  the  severe  use  of  the  bat- 
tery, /P,  owing  to  the  fact  that  it  is 
on  closed  circuit  through  the  low- 
resistance  Coil,  d,  as  long  as  the  plug 
is  in  its  socket.  In  order  to  over- 
come this  objection  the  plan  shown 
in  Fig.  157  was  devised  by  Scribner. 
The  wire  leading  from  the  thimble, 
k\  to  the  restoring  coil  is  run  to  the 
pivot  of  the  shutter,  g,  as  is  shown, 
and  the  side  of  the  restoring  coil 
which  is  not  grounded  is  run  to  the 
armature,  e.  This  connection  is 
very  clearly  shown  in  Fig.  157.  The 
shutter,  g,  is  normally  insulated  from 


Fig.  157. — Circuits  of  a  Self- 
Restoring  Drop. 


the  armature,  e,  by  virtue  of  the  small  hard-rubber  bushing,  ^5, 
and  the  insulation,/1,  between  the  bracket,/,  and  the  magnet,*/. 
This  construction  has  been  shown  in  Fig.  154.  When,  however, 
the  armature,  e,  falls  forward,  a  small  lug  or  platinum  contact 
point,  /,  on  the  upper  side  of  the  armature  strikes  against  and 
makes  contact  with  the  armature,  g,  thus  closing  the  circuit 
between  them. 

If  no  plug  is  inserted  into  the  jack  of  the  line,  the  local  circuit 
through  the  battery,  /£3,  will  be  open  at  the  jack,  and  the  arma- 
ture will  be  allowed  to  fall  forward  when  released  by  the  catch,  c\ 
As  soon,  however,  as  a  plug  is  inserted  the  local  circuit  through 


ij6  AMERICAN   TELEPHONE  PRACTICE. 

the  battery  will  be  completed,  it  being  closed  both  at  the  arma- 
ture contact  point,  j,  and  at  the  jack.  This  will  send  an  impulse 
of  current  through  the  restoring  coil,  and  pull  the  armature  back 
until  caught  by  the  catch,  c*.  This  same  movement  of  the  arma- 
ture opens  the  circuit  at  the  contact,  /,  and  thus  no  battery  power 
is  wasted.  A  subsequent  calling  signal  upon  the  line  circuit 
while  the  plug  is  in  the  jack  will  tend  to  operate  the  annunciator, 
but  almost  as  soon  as  the  shutter,  e,  is  released  the  local  circuit 
will  be  closed  at  the  contact  point,  j,  thereby  re-attracting  the 
armature  to  its  normal  condition  and  preventing  it  from  falling 
to  actuate  the  shutter. 

An  entirely  different  form  of  self-restoring  drop  has  come  into 
very  general  use  among  the  companies  operating  in  the  United 
States  in  opposition  to  the  American  Bell  Telephone  Company. 


S, 


H 

H  G 

Fig.  158. — Western  Telephone  Construction  Co.'s  Drop  and  Jack. 

These  are  what  are  termed  mechanically  self-restoring  drops,  and 
in  order  that  the  drop  may  in  each  case  be  in  close  proximity  to 
the  jack  the  two  are  usually  associated  in  one  piece  of  apparatus. 
The  combined  jack  and  drop  has,  in  some  cases,  given  good 
satisfaction  in  practice,  and  when  properly  made  possesses  some 
advantages  not  to  be  found  in  the  electrically  restoring  type  of 
drop.  In  the  first  place,  all  the  additional  coils  on  the  drops, 
and  the  additional  contacts  on  the  spring-jacks  and  the  ad- 
ditional wiring  between  the  two,  are  entirely  done  away  with. 
Another  advantage,  and  one  that  is  usually  overlooked,  is  that 
when  the  drop  falls  the  eye  of  the  operator  is  attracted  directly 
to  the  point  into  which  she  must  insert  her  plug;  while  in  the 
forms  where  the  jacks  and  the  drops  are  entirely  removed  from 
each  other  the  operator  must  first  look  at  the  drop,  ascertain  its 
number,  and  then  look  for  the  corresponding  number  of  jack  on 
the  board  below.  This  very  materially  increases  the  ease  of  op- 


SELF-RESTORING   SWITCH-BOARD   DROPS.  177 

eration,  and  consequently  tends    in    itself   to    give    more    rapid 
service. 

The  drop  and  jack  of  the  Western  Telephone  Construction 
Company  was  the  first  of  this  type  to  come  into  extended  use. 
It  is  shown  in  Figs.  158  and  159,  Fig.  158  showing  the  shutter  in 
its  normal  position,  and  Fig.  199  showing  it  after  it  has  been 
thrown  down  by  an  incoming  call.  In  these  figures  the  arrange- 


S... 


I 

Fig.  159. — Western  Telephone  Construction  Co.'s  Drop  and  Jack. 

ments  are  such  that  the  spring-jack,  J,  of  a  line  lies  directly  in 
front  of  the  actuating  coil,  E,  belonging  to  that  line,  and  the 
shutter,  C,  so  arranged  as  to  fall  directly  in  front  of  the  jack  when 
released  by  the  armature,  F.  The  combined  jack  and  drop  are 
mounted  on  a  base  of  hard  rubber,  A.  The  armature,  F,  is 
pivoted  in  the  front  head,  H,  of  the  electromagnet  by  the  pivot 
screw,  JP,  and  has  a  forwardly  extending  arm  adapted  to  support 
the  shutter,  C,  in  a  horizontal  position.  A  small  leaf  spring,  5, 
normally  holds  the  rear  end  of  the  armature  away  from  the  rear 
head,  H,  of  the  coil  and  in  a  position  to  be  attracted  by  that  head 
when  a  calling  current  is  sent  through  the  coil.  The  attraction 
of  the  rear  end  of  the  armature  causes  its  front  end  to  move  side- 
wise  and  release  the  shutter,  thus  allowing  it  to  fall  into  a  verti- 
cal position  and  display  itself  to  the  view  of  the  operator,  as 
shown  in  Fig.  1-59. 

In  order  to  make  a  connection  with  the  line  the  operator  in- 
serts her  plug  directly  against  the  shutter,  which  is  'down,  and  in 
so  doing  restores  the  shutter  to  its  normal  horizontal  position  by 
a  positive  mechanical  movement.  The  plug  is  guided  into  its 
jack  by  the  shield  or  guide-plate,  K.  In  entering  the  jack  the 
spring,  y,  is  lifted  off  the  anvil,  /,  by  the  sleeve  of  the  plug,  thus 
breaking  the  connection  through  the  coil  of  the  drop.  .  The 
spring,  J,  makes  contact  with  the  sleeve  of  the  plug,  while  the 
spring  shown  on  the  under  side  of  the  jack  makes  contact  with 


I78  AMERICAN   TELEPHONE   PRACTICE. 

the  tip  of  the  plug,  thus  continuing  the  tip  and  sleeve  sides  of 
the  line  to  the  two  strands  of  the  plug  cord. 

These  drops  and  jacks  are  mounted  into  a  sort  of  an  "egg 
case"  composed  of  the  bases,  A,  and  the  hard-rubber  side-pieces, 
B  B.  These  egg  cases  usually  contain  one  hundred  compart- 
ments, ten  wide  and  ten  high;  \\  inch  is  allowed  in  each  di- 
rection between  the  centers  of  the  jacks.  The  wires  on  which 
the  shutters  are  hung  are  common  to  each  horizontal  row  of  ten, 
and  the  other  wire  shown  is  also  common  to  each  row  of  ten. 
These  two  wires  form  the  terminals  of  the  night-alarm  circuit, 
and  when  a  shutter  is  down  the  lug,  D,  on  the  shutter  strikes 
against  the  rear  wire,  thus  making  connection  between  the  two 
and  causing  the  night  bell  to  ring. 

This  combined  jack  and  drop  has  given  very  good  service  in  a 
large  number  of  cases,  but  has  nevertheless  several  rather  serious 




D 

Fig.  1 60. — Western  Telephone  Construction  Co.'s  New  Drop  and  Jack. 

faults,  chief  among  which  may  be  mentioned  the  fact  that  the 
tip-  and  sleeve-springs  are  necessarily  very  short  and  therefore 
Jiable  to  lose  their  tension  ;  and  also  the  fact  that  the  various 
parts  of  the  jack  are  mounted  separately  upon  a  hard-rubber 
block,  and  therefore  the  entire  jack  is  not  as  rigid  as  if  all  were 
mounted  upon  a  solid  brass  block.  These  defects  have  been  to  a 
large  extent  removed  in  a  more  recent  form  of  apparatus  put  on 
the  market  by  this  company,  and  designed  by  Mr.  A.  M.  Knud- 
sen.  In  this,  shown  in  Fig.  160,  the  general  arrangement  of  the 
various  parts  is  the  same  as  in  the  type  just  described,  but  the 
springs  are  made  longer  by  mounting  them  upon  the  sides  of  the 
jack  base,  and  in  fact  making  them  continuations  of  the  frame 
itself.  The  rear  portions  of  these  springs  are  provided  with  thumb- 
screws, I  and  2,  which  pass  through  a  back  panel  in  the  board  and 
secure  the  entire  drop  and  jack  in  position,  and  at  the  same  time 
afford  means  for  connecting  with  the  tip  and  sleeve  sides  of  the 


SELF-RESTORING  SWITCH-BOARD  DROPS. 


179 


line.  The  jack-tube  A,  and  the  shield  for  guiding  the  plug  into 
the  socket  are  formed  from  a  single  casting  of  brass  firmly  secured 
to  the  jack  base,  M,  thus  providing  a  much  more  rigid  construc- 
tion than  that  shown  in  Figs.  158  and  159.  The  shutter,  E, 
operates  in  the  same  manner,  it  being  shown  in  its  exposed 

-? 


Fig.  161. — Side  Elevation  American  Drop  and  Jack. 

position,  the  path  through  which  it  swings  being  indicated  by  the 
curved  dotted  line. 

Great  sensitiveness  can  never  be  attained  with  this  drop, 
because  the  shutter  rests  upon  the  armature  rod  in  such  manner 
as  to  bear  upon  it  with  its  entire  weight.  It  can  therefore  only 
be  released  by  a  considerable  effort  on  the  part  of  the  armature, 


Fig.  162. — Top  View,  with  Coil  Removed. 

this  effort  being  due  to  the  friction  between  the  shutter  and  the 
armature  rod. 

Another  form  of  mechanically  self-restoring  drop  is  that  no 
manufactured  by  the  American  Electric  Telephone  Company. 
In  this  drop,  which  is  shown  in  Figs.  161,  162,  and  163,  the 
actuating  coil  is  mounted  directly  above  the  spring-jack.  The 
coil  is  incased  on  all  sides  except  the  top  in  a  sheet-iron  shield  or 


180  AMERICAN   TELEPHONE   PRACTICE. 

box,  b,  for  lessening  the  amount  of  induction  between  adjacent 
drops.  The  armature  of  the  magnet  is  pivoted  at  the  rear  of  this 
shield  and  carries  a  forwardly  projecting  lever,  /,  which  in  turn 
carries  on  its  forward  end  a  catch  for  holding  the  shutter,  d,  in  its 


Fig.  163. — Top  View,  with  Coil  in  Place. 

vertical  position.  On  the  shutter  is  placed  a  cam,  c,  which  when 
the  shutter  is  down  lies  in  front  of'the  opening  of  the  jack.  The 
plug  shown  in  Fig.  164,  carries  an  enlargement  or  collar,  k,  which 
collar  engages  the  cam,*:,  on  the  shutter  when  the  plug  is  inserted 
into  the  jack,  and  forces  the  shutter  into  its  normal  position. 

No  cut-out  is  provided  for  the  coil,  which  is  therefore  left  in 
series   in    the    line    during   a   conversation.     The    coil    therefore 


Fig.  164. — Plug  for  American  Drop  and  Jack. 

serves  as  a  clearing-out  drop,  and  when  so  actuated  the  cam  on 
the  shutter  falls  in  front  of  the  collar  on  the  plug.  When,  there- 
fore, the  plug  is  withdrawn  from  the  jack,  after  the  clearing-out 
signal  has  been  sent,  the  cam  again  engages  the  collar  on  the  plug 
and  the  shutter  is  restored  again  to  its  vertical  position. 

The  entire  structure  of  the  combined  drop  and  jack  is  remov- 
able from  the  board  by  taking  the  thumbnuts  off  the  screws  shown 
in  the  rear.  These  screws  pass  through  the  board  forming  the 
frame  of  the  switch-board,  and  serve  not  only  to  hold  the  jack  and 
drop  in  place,  but  to  establish  a  connection  between  the  line  wires 
and  the  line  springs  of  the  jack.  Small  springs,  g g,  on  the  back 
of  the  jack  register  with  corresponding  contacts  on  the  front  side 
of  the  backboard,  thus  serving  to  extend  the  night  alarm  and 


SELF-RESTORING   SWITCH-BOARD   DROPS.  181 

generator  circuits  from  the  jacks  to  the  other  parts  of  the  switch- 
board. By  this  means  the  proper  connections  are  automatically 
made  when  the  jack  is  slipped  in  place. 

The  drop  illustrated  in  side  elevation  in  Fig.  161  is  of  the  com- 
mon-return Vype,  and  is  therefore   provided   with  but   one   line 
terminal.     Figs.    162  and   163  show  a  later   pattern  adapted  for 
metallic  circuits  and  operating  in  the  same  general  manner  so . 
far  as  the  restoring  of  the  shutter  is  concerned.     Fig.  162  is  a 
horizontal    view   with    the   annunciator    removed,    showing    the 
arrangement  of  the  various  parts  of  the  jack.     In  this  figure  the 
coil  is  indicated   at  C  in  order  to  better  illustrate  its  circuit  con- 
nections.    Fig.  163  is  a  similar  view  of  the  complete  apparatus, 
with  the  annunciator  in  place.     The  various  .circuits  of  the  ap- 
paratus will  be  understood  most  readily  by  considering  Fig.  162. 
In  this  m  is  the  jack-tube  which  is  directly  connected  with  the 
line  terminal  screw,  L.     This  tube  is  provided  with  a  spring,  ;/, 
which  serves  to  establish  a  firmer   contact   with   the    sleeve    of 
the  plug  when  inserted  into  the  jack.     The  coil,  C,  of  the  annun- 
ciator is  connected  directly  between  the  line  terminal  screw,  L1, 
and  the  tip-spring,  /,  the  sharply  bent  portion  of  which  spring 
is  adapted  to  make  contact  with  the  tip  of  the  plug  when  inserted 
into  the  jack.     A  spring,  q,  is  connected  by  means  of  one  of  the 
small  springs, £-,  in  Fig.  161  to  one  terminal  of  the  generator.     This 
spring,  q,  is  provided  with  a  metallic  pin  which  projects  through 
a  hole  in  the  jack-tube,  m,  to  a  sufficient  distance  to  make  con- 
tact with  the  enlarged  sleeve  of  the  plug  when  the  latter  is  in- 
serted into  the  jack  to  its  fullest  extent,  but  not  far  enough  to 
engage  the  tip  contact  when  the  plug  is  in  its  normal  position  in 
the  jack.     By  this  means,  when  the  plug  is  inserted  as  far  as  it 
will  go   into  the  jack,   one   terminal   of   the    generator   is   con- 
nected    with     the     line    terminal    screw,  L,    by  means  of    the 
sleeve  spring,  n,  and  the  generator  spring,  q,  both  coming  in  con- 
tact with  the  sleeve   of  the  plug.     The  other  terminal  of  the 
generator  is  connected  with  the  spring,/,  through  the  medium  of 
one  of  the  small  contact  springs,  £-,  on  the  back  of  the  jack.  Upon 
pushing  the  plug  as  far  as  it  will  go  into  the  jack,  the  tip-spring, 
/,  rides  upon  the  insulated  portion  of  the  plug,  thus  pressing  the 
thin  spring,  s,  which   lies  parallel  with,  but  is  insulated  from,  the 
tip-spring,  into  engagement  with  the  generator  spring,/.     This 
connects  the  line  terminal  screw,  L,  with  the  generator-spring,/ 
through  the  medium  of  the  strap  conductor,  k,  and  the  calling 
current  is  therefore  sent  to  line.    It  will  be  noticed  that  the  path 
by  which  the  generator  current  passes  to  line  is  not  through  the 


1 82  AMERICAN    TELEPHONE  PRACTICE. 

coil  of  the  annunciator,  but  through  the  strap,  k,  instead  ;  and  it 
will  also  be  noticed  that  the  tip  conductor  of  the  plug  is  discon- 
nected from  the  tip-spring,/,  before  the  contact  is  made  with  the 
generator-spring,  j,  and  therefore  no  calling  current  will  pass 
back  over  the  cord  circuit  through  the  operator's  telephone. 
Upon  removing  the  pressure  from  the  plug,  a  coiled  spring  in  its 
handle  forces  it  out  of  the  jack  for  a  short  distance  until  it  as- 
sumes the  normal  or  talking  position. 

Many  other  forms  of  mechanically  self-restoring  drops  have 
been  devised,  but  the  two  types  here  described  have  come  into 
by  far  the  most  general  use. 

As  an  illustration  of  the  saving  which  either  the  electrically  or 
mechanically  self-restoring  drops  bring  about  in  the  operation  of 
switch-boards,  certain  boards  may  be  cited  that  have  been  put  into 
use  where  the  drops  are  of  the  ordinary  hand-restoring  type, 
placed  in  series  in  the  line  and  not  cut  out  by  the  insertion  of 
the  plug.  In  the  establishing  and  disestablishing  of  a  connection 
between  two  subscribers,  the  operator  was  required  to  restore  a 
switch-board  drop  four  different  times.  First  she  restored  the 
drop  of  the  line  of  the  calling  subscriber,  next  when  she  sent  a 
calling  current  to  the  line  of  the  called  subscriber  this  current 
passed  through  the  drop  of  that  line,  causing  it  to  fall.  This 
she  also  restored  by  hand,  and  lastly  when  one  or  both  of  the 
subscribers  rung  off,  the  drops  of  each  line  fell  and  were  restored 
by  hand,  thus  making  four  in  all.  Such  switch-boards  are,  of 
course,  necessarily  slow.  Moreover,  they  are  almost  invariably 
much  larger  and  more  cumbersome  than  the  more  modern  types, 
but  even  these  drawbacks  have  not  interfered  with  their  giving 
satisfactory  service  in  some  cases  in  small  exchanges. 


CHAPTER  XVII. 

COMPLETE   SWITCH-BOARDS   FOR   SMALL   EXCHANGES. 

WE  have  considered  so  far  the  circuits,  and  also  the  various 
parts,  including  drops,  jacks,  circuit  changers,  etc.,  which  go  to 
make  up  switch-boards  for  small  exchanges.  In  this  chapter 
will  be  considered  a  few  of  the  more  common  types  of  such 
switch-boards  in  their  completed  forms.  The  matter  of  properly 
constructing  the  various  parts  of  switch-boards  is  hardly  of  more 
importance  than  that  of  properly  organizing  these  parts  into  a 
complete  working  system  by  means  of  their  arrangement  in  their 
proper  circuits.  The  main  points  to  be  sought  in  the  mounting 
of  the  various  parts  of  switch-board  apparatus  so  as  to  form  a 
complete  working  organization,  are  that  the  arrangement  may  be 
such  as  to  facilitate  the  work  of  the  operator  ;  that  all  parts 
liable  to  get  out  of  order  shall  be  readily  accessible  for  repairs; 
that  all  wiring  shall  be  systematically  arranged  in  a  manner 
that  shall  preclude  the  possibility  of  short  circuits,  crosses,  or 
open  circuits;  that  the  circuits  of  the  various  lines  shall  be  free 
from  inductive  influence  upon  or  from  the  other  circuits ;  and 
that  the  framework  upon  which  the  working  parts  are  mounted 
shall  not  by  virtue  of  its  shrinking  or  warping  affect  the  proper 
operation  of  these  parts. 

In  Fig.  165  is  shown  a  front  view  of  a  loo-drop  switch-board 
manufactured  by  the  Western  Telephone  Construction  Co.  This 
board  is  designed  to  be  mounted  directly  upon  the  wall  or  upon 
a  partition  in  the  exchange.  It  is  provided  with  100  combined 
drops  and  jacks  of  the  type  shown  in  Figs.  158  and  159.  These 
drops  and  jacks  are  built  up  between  hard-rubber  partitions 
which  form  a  structure  not  unlike  an  egg  packing  case,  each  drop 
and  jack  occupying  a  separate  cell.  Below  the  line-drops  is 
placed  a  row  of  ten  clearing-out  drops  included  in  series  in  the 
tip  side  of  the  cord  circuit.  This  drop  is  provided  with  a  non- 
inductive  winding,  and  also  with  a  thin  metal  shield  of  magnetic 
material,  which  together  effectually  prevent  cross-talk  between 
two  adjacent  drops.  It  may  be  said,  however,  that  the  non-in- 
ductive winding,  while  it  accomplishes  the  object  for  which  it 
was  designed,  i.  e.,  the  elimination  of  cross-talk,  also  greatly  re- 

183 


1 84 


AMERICAN    TELEPHONE  PRACTICE. 


duces  the  efficiency  of  the  drop,  but  not  to  such  an  extent  as  to 
spoil  its  utility.  The  plugs  are  arranged  in  two  rows  of  ten  on 
the  horizontal  portion  of  the  table,  and  in  front  of  them  is  the 


Figs.  165  and  166. — "  Western  "  loo-Drop  Wall  Switch-Board. 

row   of  circuit-changing  levers,  which  operate  as    described    in 
Chapter  XV. 

The  entire  case  containing  the  line-drops  is  hinged  on  the 
framework  of  the  board,  as  is  also  the  key  table,  allowing  the 
board  to  be  opened  up,  as  shown  in  Fig.  166,  for  the  purpose  of 
facilitating  repairs.  This  figure  shows  the  method  of  cabling  the 
board,  the  line  wires  being  formed  into  ten  separate  cables,  one 
for  each  horizontal  row.*  These  cables  are  formed  of  No.  22  B. 
&  S.  guage  tinned  copper  wire  arranged  in  twisted  pairs,  the 
separate  wires  being  colored  red  and  blue.  The  insulation  of 
these  wires  is  composed  of  a  single  wrapping  of  silk  upon  which 
is  laid  two  wrappings  of  cotton.  The  various  wires  leading  to 


COMPLETE   SWITCH-BOARDS  FOR   SMALL   EXCHANGES.      185 

the  under  side  of  the  key  table  are  formed  into  a  tightly  laced 
cable  which  is  provided  with  a  knee,  as  shown  below  the  left-hand 


Fig.  167. — Circuits  of  "  Western  "  loo-Drop  Board. 

portion  of  the  key  table  in  Fig.  1 66,  which  knee  is  for  the  pur- 
pose of  allowing  a  free  movement  of  the  key  table  upon  its  hinges 


i86  AMERICAN   TELEPHONE  PRACTICE. 

without  bending  the  various  wires  of  the  cable  to  such  an  extent 
as  to  cause  breakage.  The  switch-board  cords  are  provided 
with  a  spiral  wrapping  of  wire  and  are  held  taut  by  means  of 
small  pulley  weights,  clearly  shown  at  the  bottom  of  the  figure. 

At  the  right-hand  portion  of  the  cabinet  is  placed  the  crank 
of  the  hand  generator,  the  generator  itself  being  mounted  upon  a 
shelf  within  the  switch-board  cabinet.  It  is  customary  in  most 


Fig.  168. — "  Western  "  2Oo-Drop  Table  Switch-Board. 

switch-boards  to  provide  a  hand  switch  by  means  of  which,  when 
the  power  generator  is  disabled  or  stops  running,  the  hand  gen- 
erator may  be  switched  into  circuit.  The  necessity  of  this  switch 
is  overcome  in  this  switch-board,  the  switching  operation  being 
performed  automatically  by  the  turning  of  the  hand  generator 
crank.  Thus,  while  the  hand  generator  is  not  in  use,  the  gen- 
erator terminals  of  the  ringing  keys  are  connected  with  the 
power  generator.  When,  however,  the  hand  generator  crank  is 
turned,  the  generator  terminals  of  the  ringing  keys  are  discon- 
nected from  the  power  generator  and  automatically  connected 
with  the  terminals  of  the  hand  generator.  This  arrangement  is 
ingenious  and  very  effective,  and  it  relieves  the  operator's  mind  of 
all  thought  concerning  the  position  of  the  generator  switch. 
The  circuits  of  this  switch-board  are  shown  in  Fig.  167. 


COMPLETE   SWITCH-BOARDS  FOR    SMALL   EXCHANGES.      187 

In  Fig.  168  are  shown  two  loo-drop  sections  of  this  switch- 
board mounted  upon  a  table,  in  front  of  which  either  one  or  two 
operators  may  sit.  The  apparatus,  circuits,  and  operation  of 
this  board  are  identical  with  that  of  the  boards  shown  in  Figs. 
165  and  166.  In  case  a  subscriber  whose  line  terminates  in  one 
section  calls  for  a  subscriber  w^iose  line  terminates  in  the  other 
section,  the  connection  is  made  by  reaching  across  the  face  of 
the  boards  with  the  plug  to  be  used  in  connection.  It  may  be 
said  that  this  method  of  reaching  across  may  be  used  with  suc- 
cess in  exchanges  having  as  many  as  three  or  four  hundred 
subscribers,  provided  the  boards  do  not  take  up  so  great  an 
amount  of  room  as  to  render  the  reach  too  long.  The  reach  is 
of  course  limited,  not  only  by  the  convenience  with  which  the 
operators  can  make  it,  but  also  by  the  length  of  the  cords, 
and  the  length  of  the  cords  is  necessarily  limited  by  the  height 
of  the  switch-board  above  the  floor.  As  many  as  six  of  the 
switch-boards  of  the  type  shown  in  Fig.  165  have  been  used 
side  by  side  in  this  manner,  but  of  course  better  results  would 
have  been  obtained  had  some  of  the  trunking  methods,  which 
will  be  described  later,  been  used. 

A  front  view  of  a  loo-drop  switch-board  of  the  Sterling  Elec- 
tric Company  of  Chicago  is  shown  in  Fig.  169,  and  a  rear  view 
of  the  same  board  in  Fig.  170.  In  this  the  drops  are  mounted  in 
the  panels  at  the  top  of  the  board  in  vertical  rows  of  ten  each. 
In  a  panel  directly  below  the  drops  are  the  line  jacks,  connected 
with  the  drops  by  means  of  wires  formed  into  vertical  cables  and 
shown  in  Fig.  170.  The  drops  in  this  board  are  not  properly  of 
the  self-restoring  type,  but  are  provided  with  an  attachment 
which  accomplishes  the  resetting  of  the  shutters  with  little,  if 
any,  loss  of  time.  Below  each  of  the  vertical  rows  of  drops  is  a 
knob  attached  to  a  sliding  rack  in  such  manner  that  when  it  is 
pressed  the  entire  rack  is  raised,  thus  restoring  any  shutter  which 
may  be  down  in  that  particular  vertical  row.  As  these  knobs 
are  arranged  close  to  the  spring-jacks  they  are  within  easy 
reach  of  the  hand  of  the  operator  while  she  is  inserting  a  plug. 
The  plugs  are  arranged  in  two  rows,  being  staggered  so  as  to  be 
more  easily  reached  by  the  operator.  The  panel  upon  which 
they  rest  is  of  sole-leather,  which  material  has  proven  its  ability 
to  withstand  wear,  and  at  the  same  time  offers  the  additional 
advantage  of  not  injuring  the  plugs  when  they  are  dropped 
upon  it.  The  ringing  and  listening  keys  of  the  type  shown  in 
Fig.  145  are  used  in  this  board. 

In  this  board  no  clearing-out  drops   are  used,  and  the  plugs 


1 88  AMERICAN   TELEPHONE  PRACTICE. 

are  so  arranged  that  one  in  each  pair  will  cut  out  the  line-drop 
when  inserted  in  a  jack,  while  the  other  will  not ;  this  difference 
being  brought  about  by  making  the  answering  plugs  shorter  than 
the  calling  plugs.  As  a  result,  the  drop  of  the  line  with  which 


Fig.  169. — Sterling  Electric  Co.'s  loo-Drop  Switch-Board. 

the  answering  plug  is  connected  is  left  in  circuit  to  serve  as  a 
clearing-out  drop,  while  the  drop  of  the  line  with  which  the 
calling  plug  is  connected  is  cut  out  of  the  circuit.  Another  very 
desirable  feature  of  this  board,  and  one  which  could  be  followed 
by  all  manufacturers,  is  the  means  which  are  provided  for  com- 


COMPLETE   SWITCH-BOARDS  POR   SMALL   EXCHANGES.      189 

pletely  inclosing  all  of  the  wiring  and  mechanism  of  the  switch- 
board from  behind,  so  as  to  prevent  dust  from  settling  upon 
them. 

In  Fig.  171  is  shown  the  so-called  express  switch-board  of  the 


Fig.  170. — Sterling  Electric  Co.'s  loo-Drop  Switch-Board. 

American  Electric  Co.  In  this  the  drops  and  jacks  are  of  the 
combined  self-restoring  form  shown  in  Figs.  161,  162,  and  163. 
They  are  arranged  in  ten  rows  of  ten  each,  and  by  means  of  the 
thumb-nuts  that  engage  the  line  terminal  screws  at  the  back  of 
each  drop  and  jack  are  held  in  place  within  a  cabinet.  By  the 


190  AMERICAN   TELEPHONE  PRACTICE. 

removal  of  these  thumb-nuts  any  one  of  the  combined  drops 
and  jacks  may  be  withdrawn  from  the  front  of  the  board 
without  disturbing  the"  line  connections  on  the  rear.  No  sepa- 
rate clearing-out  drops  are  used,  as  the  line  drops  serve  the 
purpose,  as  already  described.  In  front  of  the  plugs  on  the 
horizontal  table  are  arranged  the  listening  keys,  which  are  sim- 


Fig.  171. — American  Express  Switch-Board. 

lar  in  construction  to  that  shown  in  Fi'g.  149.  The  hand  genera- 
tor is  mounted  on  the  under  side  of  the  front  of  the  key  table 
within  easy  reach  of  the  operator.  The  operation  of  this  board 
is  as  follows  :  when  a  line-drop  falls  the  operator  inserts  one 
of  the  plugs  in  the  row  farthest  from  her  into  the  jack,  this 
action  automatically  restoring  the  shutter  by  means  of  the  collar 
on  the  plug  engaging  the  cam  on  the  under  side  of  the  shutter. 
The  listening  key  is  then  depressed,  in  order  to  find  out  the 
wants  of  the  subscriber  calling.  Having  found  this,  the  opera- 
tor inserts  the  mate  of  the  plug  into  the  jack  of  the  called 


COMPLETE   SWITCH-BOARDS  FOR   SMALL   EXCHANGES.      19* 

subscriber  and  presses  it  in  as  far  as  it  will  go,  at  the  same  time 
turning  the  hand  generator,  if  no  power  generator  is  used. 
This  sends  calling  current  to  the  line  of  the  called  subscriber, 
after  which  the  operator  releases  the  calling  plug  which  springs 
out  of  the  jack  to  a  sufficient  extent  to  disconnect  the  calling 
generator  and  re-establish  connection  between  the  line  and  the 
cord  circuit.  When  the  subscribers  are  through  talking  they 
ring  off,  and  the  line-drops  are.  again  actuated,  and  are  restored 
automatically  by  the  withdrawal  of  the  plugs  from  the  jacks. 


CHAPTER  XVIII. 

LAMP-SIGNAL   SWITCH-BOARDS. 

IN  all  of  the  telephone  exchange  systems  so  far  outlined  the 
various  signals  for  attracting  the  attention  of  the  operator  have 
been  given  by  some  form  or  another  of  electro-mechanical 
annunciators,  of  which  those  of  the  self-restoring  type  represent 
the  highest  development.  An  entirely  different  class  of  signals 
has  recently  come  into  general  use,  especially  by  the  Bell 
Company,  namely,  the  incandescent  lamp  or  luminous  signal,  as 
it  is  termed.  The  use  of  the  incandescent  lamp  as  a  signal  in 
telephone  work  was  probably  first  proposed  by  Mr.  J.  J, 
O'Connell  of  Chicago. 

The  advantages  of  the  incandescent  lamp  over  the  electro- 
mechanical signal  are  many,  and  among  them  maybe  mentioned 
the  following:  First,  they  are  capable  of  attracting  the  atten- 
tion of  an  operator  with  more  certainty  than  the  ordinary 
mechanical  shutter  ;  second,  they  are  entirely  free  from  mechan- 
cal  complication  ;  third,  they  are  much  more  compact  than  even 
the  simplest  electro-mechanical  signals  ;  fourth,  they  are  entirely 
automatic  in  their  operation,  being  always  restored  to  their  nor- 
mal condition  by  a  cessation  of  the  current  through  them  ;  fifth, 
by  the  use  of  various-colored  glass  in  front  of  them  they  may  be 
used  in  the  same  board  as  indicating  different  kinds  of  in- 
formation ;  sixth,  they  are  easily  replaced  when  destroyed  ;  and 
seventh,  they  are  cheaper  than  the  high-grade  tubular  drops  now 
in  such  common  use.  Against  these  advantages  must  be  cited 
the  somewhat  serious  disadvantage  brought  about  by  the  appa- 
rent inability  of  lamp  manufacturers  to  produce  a  uniform  grade 
of  miniature  lamps.  Thousands  of  lamps  furnished  to  operating 
companies  have  after  short  trial  proved  utterly  unfit  for  use.  The 
difficulty  in  procuring  good  lamps  has  in  some  cases  caused  the 
abandonment  of  the  lamp  signal  system. 

Obviously  there  are  two  different  methods  of  associating  in- 
candescent lamps  with  the  circuits  of  the  subscribers'  lines. 
The  first  of  these,  and  without  mature  consideration  the  most 
desirable,  is  to  place  the  lamp  directly  in  the  circuit  of  the  sub- 
scriber's line  and  operate  it  automatically  by  a  change  in  resist- 


LAMP   SIGNAL   SWITCH-BOARDS. 


'93 


ance  in  the  line,  brought  about  by  the  removal  of  the  subscri- 
ber's receiver  from  the  hook.  The  second  is  to  have  the  lamp 
in  a  local  circuit  controlled  by  a  relay  directly  in  the  line  circuit, 
which  relay  may  be  operated  by  an  ordinary  subscriber's  magneto- 


Fig.  172. — Lamp  Signal  System. 

generator  or  by  a  source  of  current  at  the  central  office  thrown 
into  action  by  a  change  of  resistance  at  the  subscriber's  station. 
A  system  using  the  first  method  is  shown  in  Fig.  172,  in  which 
the  subscriber's  apparatus  is  shown  at  .S,  and  the  central-office 
apparatus  at  C.  The  line  wire,  2,  forming  one  side  of  a  metallic 
circuit,  is  connected  with  the  tip-spring,  t,  of  the  jack,  y,  and 
passes  through  an  incandescent  lamp,  /,  and  through  an  induc- 
tive resistance,  ^,  to  one  pole  of  a  battery,  i.  The  other  side,  3,, 
of  the  metallic  circuit  passes  through  an  inductive  resistance,  gf 
to  the  other  pole  of  the  same  battery.  When  the  subscriber's 
receiver  is  on  its  hook  the  circuit  at  the  subscriber's  station  be- 


194  AMERICAN   TELEPHONE   PRACTICE. 

tween  the  two  sides  of  the  line  wire  is  completed  only  through 
the  high-resistance  call-bell,  e,  and  as  this  bell  has  a  resistance  of 
about  1000  ohms,  the  current  from  the  battery,  z,  through  the 
line  circuit  is  not  sufficient  to  illuminate  the  lamp,  /.  When, 
however,  the  subscriber'  s  receiver  is  removed  from  its  hook  a 
circuit  of  low  resistance  is  closed  in  parallel  with  the  bell  mag- 
nets, e,  this  circuit  including  the  secondary  winding,  <:2,  of  the 
induction  coil,  and  the  receiver,  b,  in  series.  As  this  circuit  may 
readily  be  made  less  than  40  ohms,  sufficient  current  will  be 
allowed  to  flow  from  the  battery,  z,  to  illuminate  the  signal,  and 
thus  attract  the  operator's  attention.  Another  feature  of  this 
system,  and  one  which  it  was  not  the  purpose  of  this  chapter  to 
illustrate,  but  which,  owing  to  its  ingenuity,  should  be  mentioned 
in  passing,  is  the  peculiarity  of  the  arrangement  of  the  battery, 
dy  with  respect  to  the  circuits  of  the  subscriber's  station.  It  will 
be  obvious  that  whatever  current  passes  through  the  bell  mag- 
nets, ^,  from  the  battery,  z,  at  the  central  office,  must  also  pass 
through  the  battery,  d.  This  consists  of  two  cells  of  storage 
battery  of  the  Plante  type.  Whenever  the  apparatus  at  the  sub- 
scriber's station  is  not  in  use  this  battery  will  therefore  be  re- 
ceiving a  charge  from  the  central-office  source,  the  strength  of  the 
latter  and  the  resistance  of  the  circuits  being  so  proportioned 
that  the  storage  cell  will  receive  a  constant  charging  current  of 
about  .02  of  an  ampere.  When  the  subscriber's  apparatus  is 
put  in  use,  however,  the  battery  is  thrown  in  a  local  circuit  in- 
cluding the  primary  winding,  cl,  the  transmitter,  a,  and  the  con- 
tact point,  f2,  and  will  then  perform  the  functions  of  an  ordinary 
primary  battery  in  connection  with  the  transmitter.  The  chief 
novelty  in  this  system  consists  in  the  alternative  function  which 
this  battery,  .d,  may  perform.  It  is  well  known  that  if  a  storage 
cell  of  the  Plante  type  becomes  almost  or  quite  discharged  it  will 
develop  a  counter  E.  M.  F.,  when  a  current  is  sent  through  it  in 
the  direction  necessary  to  charge  it,  and  that  this  counter  E.  M.  F. 
will  be  very  nearly  equal  to  the  E.  M.  F.  of  a  similar  cell  fully 
charged.  Supposing,  now,  that  from  some  cause  or  other  the  cell, 
d,  becomes  discharged  to  such  an  extent  that  it  is  incapable  of  fur- 
nishing enough  current  to  operate  with  the  transmitter,  a,  in  the 
usual  manner.  In  this  case,  when  the  receiver  is  raised,  the 
current  from  the  battery,  z,  at  central,  which  tends  to  pass 
through  the  storage  battery,  will  meet  with  a  considerable  coun- 
ter *E.  M.  F.,  which  will  compel  most  of  the  current  to  pass 
through  the  secondary,  c1,  receiver,  b,  wire,  4,  wire,  6,  contact,/2, 
wire,  5>  transmitter,  a,  primary  coil,  c\  and  to  the  line  wire,  3. 


LAMP   SIGNAL    SWITCH-BOARDS.  195 

The  transmitter,  a,  will  therefore  receive  current  from  the  bat- 
tery, z,  sufficient  to  operate  it,  and  yet  it  will  be  operating  with 
all  the  advantages  to  be  derived  from  a  local  circuit  and  induc- 
tion coil ;  for,  although  the  current  operating  it  comes  from 
the  central  office,  any  fluctuations  in  this  current  caused  by  the 
transmitter,  a,  will  pass  through  the  low-resistance  battery,  d, 
which  will  act  in  this  case  very  much  in  the  same  manner  as, a 
condenser.  This  system  is  the  invention  of  Mr.  C.  E.  Scribner 
of  Chicago. 

To  return  now  to  the  luminous  signal  feature,  we  find  our- 
selves confronted  with  several  rather  serious  objections;  in  the 
first  place,  the  resistances  of  no  two  subscribers'  circuits  are  the 
same,  owing  to  the  differences  in  the  lengths  of  these  circuits  and 
other  causes,  and  therefore  either  the  resistances,  g  or  h,  or  that 
of  the  bell  magnets,  e,  will  have  to  be  varied  in  each  case  in 
order  to  insure  the  proper  amount  of  current  passing  through 
the  lamp,  /.  This  is  a  feature  easy  to  overcome,  and  a  much 
more  serious  one  is  that  arising  from  crosses  between  two  line 
wires  from  any  source  whatever,  such  a  cross,  of  course,  always 
subjecting  the  lamp  to  an  undue  amount  of  current,  and  there- 
fore burning  it  out.  This  latter  objection  has  proved  so  serious 
as  to  cause  the  abandonment  of  the  plan  of  including  the  lamp 
directly  in  the  line  circuit  in  nearly  every  case  where  it  has  been 
tried.  Of  course,  for  underground  systems  this  objection  is  not 
such  a  serious  one. 

Passing  now  to  the  second  method  of  associating  the  lamp 
signal  with  the  line  circuit,  reference  will  be  made  to  Fig.  173. 
This  shows  the  circuits  of  three  subscribers'  stations,  S,  S,1  and 
52,  these  circuits  being  connected  with  the  central  office  by  the 
metallic  circuits,  I,  2.  The  line  wire,  I,  in  each  case  passes  to  the 
sleeve-spring,  <^2,  of  the  spring-jack,  y,  and  thence  through 
the  relay  contacts,  c\  to  the  ground.  The  line  wire,  2,  passes  in 
a  similar  manner  to  the  tip-spring,  d,  thence  through  the  relay 
contacts,  £2,  the  winding  of  the  relay,  b,  and  the  battery,  a,  to 
the  ground.  The  signal  lamp,  e,  is  in  each  case  included  in  a 
local  circuit  containing  the  contact,  4,  of  the  relay,  b,  belonging 
to  its  line,  and  a  battery,/",  common  to  all  lamps.  The  relay,  c, 
of  each  line,  which  controls  the  contacts,  c1  and  c*,  is  known  as 
the  cut-off  relay,  and  is  included  in  a  local  circuit  through  the 
jack-thimble,  d\  and  the  plug  contact,  m,  with  the  battery,  n, 
whenever  a  plug,  I,  is  inserted  into  a  jack  for  making  connection 
with  the  line.  The  two  sides  of  the  line  at  the  subscriber's  sta- 
tion are  permanently  closed  through  a  high-resistance  bell  and  a 


io6 


AMERICAN   TELEPHONE   PRACTICE. 


condenser,  the  latter  having  a  capacity  of  about  .75   microfarad, 
so  as  to  allow  the  alternating  currents  from  the  calling  generator 


Fig.  173-— Lamp  Relay  System. 

at  i\\G  central  office  to  pass  through  it  and  operate  the  call-bell 
in  the  usual  manner.  The  high  impedance  of  the  call-bell  mag- 
nets, however,  prevents  the  short-circuiting  of  the  voice  currents 


LAMP   SIGNAL    SWITCH-BOARDS.  197 

when  the  receiver  is  removed  from  its  hook.  The  call-bell  circuit 
therefore  presents  an  open  circuit  to  the  direct  current  from  the 
battery,  ay  thus  normally  insuring  a  condition  of  no  current  upon 
the  line  wire.  When,  however,  the  receiver  at  the  subscriber's 
station  is  removed  from  its  hook,  a  path  of  comparatively  low 
resistance  is  formed  between  the  two  line  wires,  and  a  cur- 
rent proceeds  from  the  battery,  a,  through  the  relay,  b,  contact, 
<:a,  line  wire,  2,  to  the  subscriber's  station,  back  by  line  wire, 
I,  through  relay  contact,  c\  and  by  ground  to  the  opposite  termi- 
nal of  the  battery,  a.  This  current  is  sufficient  to  operate 
the  relay,  b,  and,  unlike  the  case  where  the  lamp  was  used 
directly  in  the  line  wire,  a  considerable  variation  in  the  amount 
of  this  current  is  allowable.  The  operation  of  the  relay,  b,  closes 
the  circuit  of  the  lamp,  e,  through  the  following  path  :  from  the 
battery,/,  through  the  relay,  g,  wire,  5,  common  wire,  C,  lamp, 
e,  wire,  3,  relay  armature,  4,  and  back  by  ground  to  the  opposite 
pole  of  battery,  f.  For  the  purpose  of  clearness  the  relay,  g, 
need  not  be  considered  at  all  at  present,  as  it  has  nothing  to  do 
with  the  operation  of  the  lamp,  e,  and  we  may  therefore  consider 
one  pole  of  the  battery,/,  to  be  connected  directly  to  the  com- 
mon wire,  C.  The  closure  of  this  circuit  illuminates  the  lamp,  e. 
The  next  step  in  the  operation  of  the  system  is  the  insertion  of 
the  plug,  /,  into  the  jack  of  the  line  on  which  the  signal  is  dis- 
played. The  insertion  of  this  plug  closes  the  circuit  through  the 
relay,  c,  over  the  following  path :  from  the  battery,  n,  wire,  10, 
plug  sleeve,  m,  thimble,  dl,  wire,  9,  relay  coil,  c,  to  ground  and 
back  to  the  opposite  pole  of  the  battery,  n.  This  causes 
the  relay  to  attract  its  armature  and .  break  both  of  the 
contacts,  cl  and  c*,  thus  accomplishing  a  double  purpose,  the  first 
of  which  is  to  break  the  circuit  through  the  relay,  b,  and  thus 
cause  it  to  release  its  armature,  4,  breaking  the  circuit  through 
the  lamp,  e  ;  and  the  second  of  which  is  to  cut  off  both  sides  of 
the  line  circuit,  I,  2,  beyond  the  spring-jack,  J.  This  latter 
feature  is  a  very  important  one,  since  it  removes  all  difficulty 
from  cross-talk  and  other  troubles  in  the  auxiliary  circuits  of  the 
central  office. 

The  circuits  illustrated  in  Fig.  173  are  substantially  those  in 
common  use  by  the  Bell  Company  in  their  lamp-signal  exchanges, 
so  far  as  the  circuits  of  the  relays  and  lamps  are  concerned.  Sev- 
eral different  modifications  of  the  circuits  at  the  subscribers'  sta- 
tions have,  however,  been  used.  The  subscribers  stations  may 
or  may  not  include  local  batteries,  and  the  tendency  of  prac- 
tice is  now  to  do  away  with  these  entirely  and  to  supply  current 


198  AMERICAN    TELEPHONE   PRACTICE. 

from  central  office  for  the  operation  of  the  transmitters  as  well  as 
for  all  signaling  purposes.  This  feature,  however,  will  form  the 
subject  of  a  subsequent  chapter. 

The  relay,  g,  and  its  associated  apparatuses  form  a  very  inter- 
esting addition  to  the  system  as  outlined.  It  is  found  desirable 
to  use  what  are  termed  pilot  lamps  for  certain  groups  of  line 
lamps,  for  the  purpose  of  attracting  the  operator's  attention  more 
surely  to  a  signal  on  her  board.  If  two  or  more  signals  are 
displayed  at  once,  but  one  of  them  may  attract  the  attention 
of  the  operator,  who  might  therefore  neglect  the  other. 
Pilot  lamps  are  used  in  such  connection  that  they  remain 
lighted  as  long  as  any  one  of  the  line  lamps  in  their  group  is 
lighted,  and  as  they  occupy  a  very  conspicuous  position  and 
are  as  a  rule  brighter  than  the  others  they  cannot  escape 
the  operator's  attention.  It  is  not  desirable  to  put  a  relay 
for  operating  such  a  pilot  lamp  in  the  common  wire,  C, 
of  the  local  circuits  of  the  lamps,  ^,  for  the  reason  that 
the  fall  of  potential  or  drop  through  such  a  relay  would  vary, 
according  to  the  amount  of  current  passing  through  it,  and  if 
several  of  the  lamps,  e,  were  operating  at  the  same  time,  they 
would  probably  not  therefore  receive  enough  current  to  properly 
illuminate  them.  The  relay,  g,  is  therefore  included  in  circuit 
with  battery,  y,  and  means  are  provided  whereby  its  resistance 
will  be  short-circuited  the  instant  it  is  operated.  To  accomplish 
this,  the  armature,  g1,  makes  contact  with  the  point,  g3,  as  soon 
as  it  is  attracted,  thus  short-circuiting  the  resistance  of  the  coil, 
g.  At  the  same  time  it  breaks  contact  with  the  point,  g*,  and 
thus  allows  the  current  to  flow  from  the  battery,/",  through  the 
lamp,  it  and  resistance,  k,  thus  illuminating  the  lamp.  In  order 
that  the  armature,  g,  may  not  fall  back  against  the  contact  point, 
g*,  as  soon  as  the  coil,  d,  is  de-energized  by  being  short-circuited, 
a  small  dash  pot,  k,  is  provided  in  connection  with  the  armature 
to  render  its  movements  sluggish.  Thus  before  the  armature,  g\ 
has  time  to  move  very  far  away  from  the  point,  ^  owing  to  its 
sluggish  action,  it  will  be  at  once  attracted  again,  and  the  inter- 
val during  which  the  resistance  coil,  g,  is  in  circuit  with  the 
common  wire,  C,  and  the  lamps,  e,  is  so  small  that  it  does  not 
have  time  to  affect  these  lamps.  This  system  is  also  due  to  Mr. 
Scribner,  and  both  it  and  the  one  shown  in  Fig.  172  form  inter- 
esting examples  of  the  highly  skillful  manner  in  which  he  always 
solves  his  telephone  problems. 

Incandescent  lamps  for  signaling  purposes  are  commonly  built 
for  10  or  20  volts  pressure,  the  tendency  being  rather  to  increase 


LAMP   SIGNAL    SWITCH-BOARDS.  199 

the  voltage  than  to  decrease  it.  At  first  lamps  of  2  and  4  volts 
were  used,  but  for  various  reasons,  not  the  least  among  which 
was  the  trouble  of  securing  proper  contacts  at  the  relay  and 
switch  points  for  such  low  voltages,  the  voltage  was  gradually  in- 
creased to  the  above-mentioned  figures. 

Mr.  A.  V.  Abbott  of  Chicago  has  recently  given  some  interest- 
ing figures  concerning  the  life  of  incandescent  lamps  in  switch- 
board work,  and  mentions  one  case  where  a  lamp  was  flashed 
over  a  million  times  without  showing  serious  signs  of  deteriora- 
tion. His  tests  seem  to  indicate  that  for  general  service  in 
switch-board  work  the  average  lamp  will  live  for  a  period  of  about 
1 200  hours,  although  in  laboratory  tests  a  much  longer  life  has 
proved  possible.  He  points  out,  as  a  result  of  his  observations, 
that  according  to  theory,  the  lamps  used  in  subscribers'-line 
circuits  should  last  about  twenty-five  years,  and  those  in  the  cord 
circuits  used  as  "  supervisory "  and  clearing-out  lamps,  from 
one  to  two  years.  He  also  says  that  such  a  life  has  already  been 
obtained  in  the  cord-circuit  lamps,  but  that  it  is  doubtful  if  the 
theoretical  limit  for  the  line  lamps  will  ever  be  closely  approxi- 
mated. 


CHAPTER  XIX. 

THE    MULTIPLE    SWITCH-BOARD. 

WHEN  the  number  of  subscribers  in  an  exchange  exceeds  400 
or  500,  the  switch-boards  so  far  considered  become  inadequate ; 
for  in  order  to  afford  room  for  the  number  of  operators  needed 
to  properly  handle  all  the  connections,  the  board  must  be  made 
of  considerable  width,  and  is  thus  too  wide  for  the  operators  to 
reach  across  with  their  cords. 

The  multiple  switch-board,  which  is  designed  to  enable  each 
operator  to  make  any  connection  required  without  the  aid  of  any 
other  operator,  and  without  the  use  of  unduly  long  cords,  is 
used  in  most  of  the  large  exchanges  in  this  and  other  countries. 
The  idea  underlying  the  construction  of  multiple  boards  is  very 
simple.  In  practice,  however,  the  greatest  complexity  is  met, 
but  this  is  due  entirely  to  the  great  number  of  repetitions  of 
one  comparatively  simple  circuit. 

The  boards  are  divided  into  sections,  each  section  usually 
affording  working  room  for  three  operators.  Each  line,  instead 
of  being  provided  with  a  single  spring-jack  or  terminal,  as  on 
the  boards  used  in  small  exchanges,  is  provided  with  a  spring- 
jack  on  every  section  of  the  board,  and  with  a  drop  or  other  visual 
signal  on  one  section  only.  Each  section  therefore  contains  a 
spring-jack  for  every  line  entering  the  exchange,  and  also  a 
number,  usually  200,  of  line-drops.  Suppose  an  exchange  to 
have  3000  subscribers.  The  multiple  board  would  then  prob- 
ably have  15  sections,  each  containing  3000  jacks,  that  is,  a  jack 
for  each  line.  Each  section  would  also  contain  200  drops  belong- 
ing to  the  200  lines  whose  calls  would  always  be  received  on  that 
particular  section.  An  additional  jack,  called  an  answering  jack, 
is  usually  provided  for  each  line  on  the  particular  section  at 
which  that  line's  drop  is  located.  These  answering  jacks  are 
placed  in  a  separate  panel  at  the  lower  part  of  the  switch-board. 

Before  considering  any  particular  form  of  multiple  board 
it  is  probably  well  to  describe  in  a  general  way  the  opera- 
tion of  the  multiple  board.  When  a  subscriber  calls  the  at- 
tention of  the  operator  at  whose  section  his  drop  is  located, 
the  operator  plugs  into  the  answering  jack  of  that  line  with  an 


THE  MULTIPLE   SWITCH-BOARD.  2oi 

answering  plug,  and  having  switched  her  telephone  into  the  cord 
circuit  of  that  plug,  ascertains  the  number  of  the  subscriber 
desired.  She  then  completes  the  connection  with  the  subscriber 
called  for  by  inserting  the  calling  plug  into  the  multiple  jack  of 
that  subscriber's  line,  one  of  which  is,  of  course,  on  her  section. 

As  each  section  contains  one  multiple  jack  for  every  line  in 
the  exchange,  it  is  evident  that  an  operator  will  always  be  able 
to  complete  a  connection  with  any  subscriber  who  may  be  called 
for  by  any  of  the  200  subscribers  whose  drops  are  located  at  her 
section.  During  the  least  busy  portions  of  the  day  one  operator 
at  each  section  usually  suffices  to  handle  all  of  the  calls  originat- 
ing at  that  section.  As  the  number  of  calls  increases  two 
operators  may»  be  placed  at  each  section,  and  during  the  busiest 
part  of  the  day  three  are  usually  required. 

When  three  operators  are  seated  at  a  section,  the  center  one 
can  reach  all  of  the  jacks  on  the  section  at  which  she  works. 
The  operator  at  her  right  cannot  well  reach  the  jacks  on  the 
extreme  left-hand  portion  of  that  section,  but  she  has  within  her 
reach  a  similar  portion  of  the  section  at  her  right  into  which  she 
may  plug  when  necessary.  In  a  similar  manner,  the  operator  at 
the  left  cannot  well  reach  the  jacks  on  the  extreme  right  of  her 
of  her  own  section,  but  can  reach  with  her  left  hand  the  jacks  on 
the  extreme  right  of  the  section  at  her  left.  Thus  every  opera- 
tor has  a  multiple  jack  for  each  of  the  entire  number  of  lines 
within  her  reach  ;  the  right-hand  operator  controlling  the  right- 
hand  two-thirds  of  her  own  section,  and  the  left-hand  one-third 
of  the  section  at  her  right,  the  center  operator  controlling  her 
•entire  section,  and  the  left-hand  operator  controllingt  the  left- 
hand  two-thirds  of  her  section,  and  the  right-hand  one-third  of 
the  section  at  her  left. 

In  order  to  prevent  two  or  more  connections  being  made  to  one 
line  at  different  boards,  some  sort  of  a  test  system  is  necessary. 
It  is  therefore  usually  so  arranged  that  when  a  line  is  "busy"- 
that  is,  when  it  is  connected  to  some  other  line  for  conversation — 
an  operator  at  some  other  board  than  the  one  at  which  the 
connection  is  made,  in  trying  to  connect  another  party  to  that 
line,  will  in  some  simple  way  be  notified  of  the  fact  that  that  line 
is  already  busy.  This  is  known  as  the  "  busy  test "  and  on  its 
efficiency  to  a  great  extent  depends  the  operativeness  of  the 
multiple  board. 

In  Fig.  174  are  shown  diagrammatically  three  lines  passing 
through  three  separate  sections  in  a  multiple  board.  One 
side  of  each  line — for  instance,  of  line  I — passes  in  multiple 


202 


AMERICAN    7'ELEPHONE   PRACTICE. 


to  all  the  contact  rings,  b,  of  a  jack  on  each  section.  It  then 
passes  to  one  terminal  of  the  line-drop,  d.  The  other  side  of 
the  line  passes  to  the  spring,  a,  in  the  jack  belonging  to  that  line 
at  section  I.  This  spring  rests  against  an  anvil,  £,  to  which  a 
wire  is  connected  which  runs  to  spring,  a,  of  the  jack  belonging 
to  that  line  on  the  second  section.  The  anvil  from  this  jack  is 
connected  to  the  line  spring,  a,  of  the  jack  on  the  third  board, 
and  so  on  the  connection  of  the  line  is  continued  through  a  jack 


M 

2,vne,  2.. 

Line  3. 

.c 

a\ 

\ 

> 

b 

j 

[ 

S 

\> 

\ 

HJ      fij       fij 

o"   o"   o"        a"  o"   a: 

fi 

r 

a  1 

i 

Fig.  174. — Simplified  Diagram  of  Series  Multiple  Board. 

on  every  section,  and  finally  to  the  other  terminal  of  the  drop,  d. 
Lines  2  and  3  pass  successively  through  the  sections  in  a  similar 
manner. 

When  a  subscriber  operates  his  generator,  the  current  passes 
over  the  line  wire  through  all  of  the  contacts,  a  and  ^,  in  series, 
through  the  drop-coil,  and  back  over  the  other  side  of  the  line. 
When  an  operator  inserts  a  plug  into  a  jack,  the  spring,  a,  is 
lifted  from  contact  with  the  anvil,  c,  by  the  tip  of  the  plug.  The 
sleeve  of  the  plug  makes  connection  with  the  test-ring,  b,  and 
thus  the  tip  and  sleeve  strands  of  the  plug  are  connected, 
respectively,  into  the  metallic  circuit  of  the  line,  while  the  circuit 
through  the  drop  is  cut  off  at  the  anvil,  c. 

The  operator's  telephone,  T,  may  be  then  bridged  across  the 
cord  circuit  in  order  to  enable  the  operator  to  converse  with  the 


THE   MULTIPLE   SWITCH-BOARD. 


203 


subscriber  who  has  called.  Means  for  connecting  the  operator's 
telephone  in  the  circuit  in  this  manner  are  not  shown  in 
Fig.  174,  the  details  of  the  cord  circuit  being  described  later 
in  connection  with  another  figure.  This  telephone  in  Fig.  174  is 
shown  connected  in  a  ground  branch  from  the  tip  side  of  the 
cord  circuit,  in  order  to  better  illustrate  the  principles  of  testing 
in  this  system. 

The  sleeve  strand  of  each  cqrd  circuit  is  grounded  through  a 
battery,  B,  and  in  order  that  this  ground  may  not  produce  serious 


175.  —  Cord  Circuit  of  Series  Multiple  Board. 


effects  in  unbalancing  or  crossing  the  circuit  of  two  connected 
lines,  an  impedance  or  reactance  coil,  /,  is  placed  in  this  circuit. 
Whenever  any  plug  is  inserted  into  a  jack,  one  side  of  the  test- 
battery,  B,  is  thrown  on  to  all  of  the  test-rings,  b,  of  the  line  to 
which  that  jack  belongs.  If  now  an  operator  at  another  board 
desires  to  make  a  connection  with  that  line,  she  touches  the  tip 
of  her  answering  plug  to  the  test-ring,  b,  of  that  line.  This  will 
connect  the  test-ring,  b,  to  ground,  through  her  telephone,  T,  and 
a  click  will  be  heard,  due  to  the  passage  of  the  current  from 
battery,  B.  The  operator  will  therefore  know  that  that  line  is 
busy,  and  will  refrain  from  making  the  connection. 

In  Fig.  1  74  the  three  lines  have  their  drops  located  at  section 
3.  It  must  be  remembered  that  other  lines  would  pass  through 
jacks  on  the  various  sections  in  a  similar  manner,  but  would  have 
their  drops  located  on  sections  I  or  2.  The  operator  at  any  sec- 
tion will,  of  course,  answer  calls  on  lines  terminating  or  having 
drops  on  her  section  only,  but  she  may  be  required  to  connect 
one  of  these  lines  to  any  other  line  in  the  exchange  by  means  of 
the  multiple  jack. 

The  details  of  the  cord  circuit  for  this  system  are  shown  in 


204  AMERICAN    TELEPHONE   PRACTICE. 

Fig.  175.  K  and  K'  are  ringing  keys  for  connecting  the  genera- 
tor, G,  with  either  of  the  plugs,  P  or  P.  The  circuit  between 
the  plugs  is  normally  maintained  continuous,  through  the  tip 
and  sleeve  strands,  as  can  be  readily  seen.  When  the  listening 
key,  K' ',  is  depressed,  the  condenser,  C,  is  looped  into  the  tip 
strand  and  at  the  same  time  the  operator's  telephone  circuit  is 
bridged  between  the  tip  and  the  sleeve  strand.  The  center  point 
of  the  coil  of  the  operator's  receiver,  R,  is  grounded,  and  the 
secondary  coil  is  split  into  two  parts,  5  and  S',  one  part  on  each 
side  of  the  receiver.  This  arrangement  is  to  prevent  the  unbal- 
ancing of  the  line  by  the  ground  on  the  receiver  coil.  The  test 
is  made  when  the  key,  K",  is  depressed,  the  test  circuit  then  being 
from  the  the  tip  of  the  plug,  P',  through  the  tip  strand  to  the 
right-hand  spring  of  the  key  and  through  its  anvil,  the  part  5  of 
the  secondary  coil,  and  one-half  of  the  receiver  coil  to  ground. 
The  condenser,  C,  is  for  the  purpose  of  preventing  disturbances 
in  the  line  with  which  the  plug,  P,  is  connected  from  giving  a 
false  busy  test. 

The  arrangement  of  circuits  here  shown  is  that  used  in  what  is 
termed  the  series-multiple  board,  the  name  series  being  derived, 
of  course,  from  the  manner  in  which  the  line  passes  through  the 
contact-springs  and  anvils  of  the  multiple  jacks.  This  system, 
although  once  widely  used,  is  subject  to  grave  defects,  and  is 
being  rapidly  replaced  by  another  form  of  multiple  board  known 
as  the  ''bridging"  or  "branch  terminal  multiple."  In  the  series- 
multiple  an  open  circuit  may  be  caused  in  any  one  of  the  jacks 
by  a  particle  of  dust  or  other  foreign  insulating  matter  becoming 
lodged  between  the  line-spring  and  its  anvil,  or  by  virtue  of  one 
of  the  springs  becoming  weak  and  failing  to  bear  upon  its  anvil. 
The  liability  to  open  circuits,  therefore,  is  very  great,  especially 
in  large  exchanges. 

Another  serious  objection  to  the  series  board  is  that  when  a 
plug  is  inserted  into  a  jack,  one  side  of  the  line  is  cut  off  at  the 
anvil  of  that  jack,  but  the  test  side  is  not  cut  off,  and  is  continu- 
ous through  the  drop  of  that  line  and  back  to  the  anvil  of  the 
jack  which  is  plugged.  This,  in  a  large  exchange,  means  that  to 
one  side  of  the  line  is  attached  an  open  branch,  perhaps  several 
hundred  feet  long  and  containing  the  drop-coil.  This  destroys 
to  a  certain  extent  the  balance  of  the  line,  and  is  liable  to  pro- 
duce cross-talk. 

The  branch-terminal  system  was  designed  to  remedy  the 
defects  inherent  in  the  series  system,  and  possesses  many  advan- 
tages over  it,  chief  arnon^  which  are  the  facts  that  when  a  con- 


THE  MULTIPLE   SWITCH-BOARD. 


205 


nection  is  made  with  any  line  the  balance  of  that  line  is  in  nowise 
affected,  and  that  the  liability  of  open  contacts  in  the  jacks, 
which  is  such  a  serious  defect  in  the  series  system,  does  not  exist. 
The  branch-terminal  system,  moreover,  lends  itself  more  readily 
to  the  use  of  self-restoring  drops,  as  will  be  described  later. 

In   Fig.   176  is   represented  one   type   of  the  branch-terminal 
system,  sometimes  called  the  three-wire  system.     In  this  figure 


Fig.  176. — Diagram  of  Branch  Terminal  Multiple  Board. 

three  distict  line  circuits  are  shown  passing  through  three  sections 
of  board.  The  wires,  k  and  k' ,  of  each  line  have  branch  wires 
leading  off  to  a  jack  on  each  board  ;  the  branches  from  wire,  k, 
leading  to  the  contact-thimbles,/",  and  the  branches  from  wire, 
k\  leading  to  the  short  springs,  c,  in  the  same  jacks.  Bridged 
across  the  two  wires  of  each  line  is  the  line  coil,  n ',  of  the  indi- 
vidual annunciator  belonging  to  that  line.  This  coil  is  high- 
wound  in  order  that  it  may  be  left  permanently  bridged  across 
the'line  without  materially  affecting  the  efficiency  of  the  system 
in  talking. 

A  third  wire,  /,  passes  through  the  board  in  parallel  with  each 
line.  From  this  wire  branch  wires  are  run  to  the  test-thimble,^, 
and  to  the  spring,  b\  in  each  jack  belonging  to  that  line.  The 
test  wire,  /,  after  passing  through  all  of  the  boards,  runs  through 
a  low-resistance  coil,  «2,  on  the  drop  of  the  line  to  which  that 
particular  test  wire  belongs,  and  then  passes  to  ground  through 
a  battery,  o,  common  to  all  test  wires.  These  test  wires  are 
represented  by  dotted  lines  in  the  figure  in  order  to  distinguish 


206  AMERICAN    TELEPHONE   PRACTICE. 

them  more  readily  from  the  line  wires.  The  remaining  spring,  b, 
in  each  jack  is  permanently  connected  to  a  ground  wire,  G,  com- 
mon to  all  of  the  jacks. 

Each  plug  in  this  system  is  provided  with  two  contacts,  h  and/, 
which  form  terminals  respectively  of  the  sleeve  and  tip  strands  of 
the  cord  circuit.  The  tip,  //,  registers  with  the  spring,  c,  when 
the  plug  is  inserted  into  the  jack  (see  Fig.  177),  and  the  sleeve,/, 


Fig-  177.— Three-Wire  Plug  and  Jack. 

registers  with  thimble,  /.  A  conducting  ring,  t,  entirely  insulated 
from  all  other  portions  of  the  plug,  registers  with  the  springs,  b 
and  &',  in  the  jack,  and  connects  them  together  electrically. 

Three  general  results  are  accomplished  by  the  insertion  of  a 
plug  into  a  jack.  The  tip  and  sleeve  strands  of  the  cord  circuit 
are  connected  respectively  with  the  sides,  k  and  k,  of  the  line, 
thus  continuing  the  line  circuit  to  the  cord  circuit.  The  con- 
necting of  springs,  b  and  b ',  by  the  ring,  i,  completes  the  circuit 
of  the  battery,  <?,  through  the  restoring  coil,  n*,  of  the  annunciator, 
to  the  ground  wire,  G,  and  thus  allows  current  from  this  battery 
to  energize  the  coil,  ;/2,  and  restore  the  shutter  of  the  annunciator. 
Lastly,  the  connecting  of  springs,  b  and  b ',  by  the  ring,  z,  connects 
the  test-thimble,  g,  to  ground  by  a  short  circuit,  so  that  when  an 
operator  at  any  other  board  touches  the  test-thimble  of  that  line 
with  the  tip  of  her  plug,  a  signal  will  be  given  denoting  the  line 
as  busy. 

In  the  normal  or  idle  condition  of  a  line,  the  test-ring,  £-,  is 
electrified  to  a  difference  of  potential  from  the  earth  by  the 
battery,  o,  which  finds  circuit  through  the  restoring  coil,  wa,  of  the 
annunciator  of  that  line  to  all  the  different  test-rings,  g,  belong- 
ing to  that  line  at  all  of  the  sections  of  the  board.  If  when  the 
line  is  in  that  condition  the  tip  of  the  test-plug,  which  is  grounded 
through  the  operator's  receiver  and  the  same  battery,  be  applied 
to  test  ring,  no  current  will  flow  through  the  receiver  because 
both  the  tip  and  the  test-ring  are  at  the  same  potential.  Silence 
will  therefore  indicate  a  free  line. 

When,  however,  the  line  has  been  put  into  use  by  the  insertion 
of  a  plug  into  the  spring-jack  thereof,  the  springs,  b  and  b ',  are 
connected  by  the  contact-ring,  2,  carried  on  the  plug,  whereby 
all  the  test-thimbles,  g^  belonging  '  to  that  line  are  connected 
directly  to  earth  through  a  short  circuit,  and  therefore  no 


THE  MULTIPLE   SWITCH-BOARD. 


207 


difference  of  potential  exists  between  them  and  the  earth. 
Thus,  when  a  test  is  made  on  a  spring-jack  of  that  line  there  will 
be  a  flow  of  current  through  the  operator's  receiver  to  ground, 
and  a  click  will  be  the  result. 

Fig.  176  is  stripped  of  all  unnecessary  detail  in  order  to  enable 
the  general  underlying  principles  to  be  more  readily  grasped. 
In  Fig.  178  the  same  system  is  shown  more  in  detail  as  to  circuits, 


Fig.  178. — Complete  Circuits  of  Branch  Terminal  Multiple  Board. 

connections,  and  apparatus.  Fig.  176  will  give  the  reader  a  better 
understanding  of  how  the  jacks  are  grouped  into  sections,  and  of 
the  relative  location  of  the  parts,  while  Fig.  178  will  enable  a 
better  study  of  the  circuits. 

In  this  figure  two  subscribers,  I  and  2,  are  shown  connected 
by  line  wires,  k  and  k ',  with  the  exchange.  Jacks  /  and  /',  at 
sections  I  and  2  of  the  board,  are  shown  in  connection  with  the  line 
leading  from  station  I.  Jacks  r  and  /'-are  shown  connected  at 
the  same  sections  with  the  line  leading  from  station  2.  Across 
the  line  leading  from  station  I  is  bridged  the  line  coil,  «',  of  the 
annunciator,  this  annunciator  being  placed  at  section  2  of  the 
board.  The  line  coil  of  the  annunciator  of  line  2  is  similarly 
bridged  across  the  two  sides  of  the  line,  and  is  placed  at  section  I 
of  the  board.  A  little  study  will  show  that  the  circuit  of  the  line 
wires  and  the  test  wires  are  the  same  in  Figs.  176  and  178, 


208  AMERICAN    TELEPHONE   PRACTICE. 

although    represented    in    an    entirely    different    manner.     Like 
letters  correspond  to  like  parts  in  these  two  figures. 

Two  pairs  of  connecting  plugs  and  their  accessory  appliances 
are  shown  complete,  one  at  each  section  of  the  switch-board. 
The  tips  of  the  two  plugs  of  a  pair  are  connected  together  by 
one  of  the  conductors,  q,  of  the  flexible  cord,  and  the  sleeves,/, 
are  likewise  connected  by  the  conductor,  q' ,  of  the  same  cord. 
Included  in  circuit  between  the  two  plugs  of  a  pair  are  two  call- 
ing keys,  r  and  r',  each  adapted  to  disconnect  both  contact-pieces 
of  one  of  the  plugs  from  those  of  the  other,  and  to  connect  them 
to  the  anvils,  s  and  s',  which  form  the  terminals  of  the  calling 
generator,  P,  G. 

A  listening  key,  u,  is  provided  for  each  cord  circuit,  having  con- 
tact points  or  anvils  connected  with  the  conductors,  q  and  q  y  as 
shown,  and  having  its  contact-spring,  u  and  w2,  connected  with 
the  terminals  of  the  operator's  telephone,  w.  When  the  plunger 
of  the  listening  key,  u,  is  allowed  to  rise,  the  operator's  telephone 
is  connected  in  a  bridge  across  the  two  sides  of  the  cord  circuit, 
as  is  shown  at  section  I.  A  wire  is  connected  from  the  middle 
point  of  the  coil  of  the  operator's  telephone  receiver  to  ground 
through  the  battery,  o,  so  that  when  a  test  is  made  of  any  line,  as 
was  described  above,  a  circuit  will  be  completed  from  the  contact- 
thimble,  gy  of  the  jack  through  the  tip  strand,  q,  of  the  cord  cir- 
cuit, and  thence  through  one-half  of  the  operator's  receiver  coil  to 
the  ground.  As  this  wire  leads  from  the  center  part  of  the  opera- 
tor's receiver  coil,  it  maybe  left  connected  permanently,  as  it  does 
not  destroy  the  balance  of  the  line. 

A  clearing-out  annunciator,  xt  similar  in  construction  to  the 
line  annunciator,  has  its  high-resistance  coil,  x',  bridged  perma- 
nently across  the  two  sides  of  each  cord  circuit.  The  restoring 
coil,  x*,  is  connected  in  a  normally  open  local  circuit,  including 
the  battery,  <?,  and  terminating  in  the  ground  on  one  side,  and  in 
a  spring,  u3,  on  the  other.  This  spring,  u3,  is  arranged  in  con- 
junction with  the  listening  key  in  such  a  manner  that  when  the 
key  is  raised  the  spring  will  make  contact  with  a  grounded  anvil, 
d.  Thus,  whenever  the  operator  listens  in  on  any  cord  circuit  she 
at  the  same  time  restores  the  clearing-out  drop  if  it  happens  to  be 
down. 

In  order  to  give  the  reader  a  clearer  understanding  of  the  sys- 
tem so  far  described,  it  will  be  well  to  follow  the  operation  in 
connecting  one  subscriber  with  another.  Suppose  Subscriber  I 
desires  connection  with  Subscriber  2.  He  operates  his  generator,. 
i,  and  the  current  therefrom  passes  over  the  line  wires,  k  k' ,  and 


IVEBST  ' 

CAUF02!S> 
THE   MULTIPLE   SWITCHBOARD.  209 

through  the  coil,  ri,  of  the  line  annunciator,  n,  at  section  2  of  the 
board.  The  operator,  noticing  this  signal,  inserts  plug, /,  into 
jack,-/'.  This  completes  the  circuit  from  ground,  through  bat- 
tery, o,  coil,  ft*,  of  the  line  annunciator,  thence  to  spring,  b', 
through  the  ring,  z,  on  the  plug  to  spring,  b,  and  to  ground. 
The  front  armature  of  the  annunciator  is  therefore  attracted  and 
the  drop  restored. 

The  operator  then  connects  her  telephone  across  the  cord  cir- 
cuit by  raising  the  key,  u,  and  communicates  with  Subscriber  No. 
I,  in  order  to  ascertain  his  wishes.  Having  found  that  he  desires 
a  connection  with  Subscriber  No.  2,  she  takes  up  plug,/',  of  the 
same  pair  and  tests  to  find  out  whether  line  No.  2  is  con- 
nected to  at  some  other  board.  If  it  is  busy  a  current  will  pass 
from  battery,  o,  through  one-half  of  the. coil  of  her  receiver,  and 
one  part  of  the  secondary  to  the  spring,  u ',  in  the  listening  key~ 
From  this  spring  it  passes  to  the  tip  strand,  q,  of  the  cord  circuit,, 
and  to  the  tip,  h,  of  the  testing  plug.  As  the  test-thimble  to 
which  the  plug  is  applied  is  grounded  by  the  insertion  of  a  plug 
at  another  board,  the  current  will  pass  through  it  to  ground. 
This  will  produce  a  click,  which  will  indicate  to  her  that  the  line 
is  busy,  and  she  will  not  complete  the  connection  called  for.  If? 
however,  she  finds  the  line  to  be  free  she  thrusts  the  plug  entirely 
into  the  jack,  in  which  position  it  is  shown  in  the  figure,  and 
depresses  the  key,  r\  in  order  to  throw  current  from  generator 
Pj  G,  upon  the  line  of  Subscriber  No.  2. 

The  two  subscribers  are  now  connected  for  conversation. 
When  either  rings  off  the  current  passes  through  the  coil,  x' y 
bridged  across  the  cord  circuit,  and  actuates  the  clearing-out  drop. 
The  operator,  noticing  this,  again  listens  in,  by  raising  the  key, 
//,  in  order  to  find  out  whether  they  are  through  talking,  or 
whether  one  of  them  desires  another  connection.  The  act  of 
listening  in  closes  spring,  &3,  against  anvil,  d,  and  thus  restores  the 
shutter  of  the  clearing-out  drop.  If  the  subscribers  have  finished 
talking,  the  plugs  are  removed  and  placed  in  their  normal  resting- 
place. 

If,  while  Subscribers  I  and  2  were  connected  together  at  sec- 
tion 2,  as  above  described,  someone  at  section  I  had  desired  con- 
nection with,  say,  line  No.  2,  the  operator  at  section  i,  in  applying 
the  tip  of  her  plug,  /3,  to  the  test-thimble,  g,  as  shown,  would 
receive  a  click  in  her  receiver  for  the  reason,  as  pointed  out  above, 
that  contact,  g,  is  connected  to  the  ground  by  a  short  circuit  by 
the  plug  inserted  in  jack,  /s.  No  difference  of  potential  would, 
therefore,  exist  between  thimble,  g,  and  the  ground,  and  hence  a 


210 


AMERICAN    TELEPHONE   PRACTICE. 


current  from  the  battery,  o,  would  pass  through  the  telephone  of 
the  operator  making  the  test. 

Success  in  practical  telephone  working  can  be  attained  only  by 
the  greatest  attention  to  matters  of  detail.  Nowhere  is  this  fact 
better  illustrated  than  in  the  design  of  the  various  parts  which  go 
to  make  up  a  multiple  board.  In  the  construction  of  large  boards 


Fig.  179. — Plan  View  of  Multiple  Jack-Strip. 

of  this  type,  the  possible  capacity  of  the  board  is  limited  by  the 
number  of  spring-jacks  that  can  be  placed  within  the  reach  of  a 
single  operator.  It  is  evident,  therefore,  that  space  must  be 
economized  to  the  last  degree,  and  yet  the  jacks  must  be  sub- 
stantial, in  order  to  resist  the  wear  and  tear  of  years  of  service  ; 
must  be  made  easily  removable  so  as  to  be  accessible  for  repairs ; 
must  perform  their  electrical  functions  with  absolute  certainty, 
and  at  the  same  time  be  so  arranged  as  to  facilitate  the  orderly 


Fig.  1 80. — Front  and  Rear  View  of  Multiple  Jack-Strip. 

and  systematic  connection  of  the  wires  leading  from  the  line 
cables. 

Moreover,  when  we  consider  that  a  multiple  board  with  a 
capacity  of  5000  subscribers  will  have  in  the  neighborhood  of  130,- 
ooo  spring-jacks,  we  can  easily  realize  that  the  cost  of  produc- 
ing these  jacks  must  be  seriously  considered.  It  is  well  to  state 
here,  however,  that  any  economy  in  the  construction  of  a  switch- 
board that  will  tend  to  decrease  its  durability  and  reliability  of 
action  is  poor  economy  indeed. 

As  an  illustration  of  modern  spring-jack  construction",  we  will 
consider  the  spring-jacks  used  in  the  branch-terminal  multiple 


THE   MULTIPLE   SWITCH-BOARD. 


211 


board  just  described.  It  has  become  common  practice  to  mount 
the  jacks  in  strips  of  twenty,  and  to  so  arrange  each  strip  that  it 
may  be  removed  from  the  board  by  the  removal  of  two  screws, 
which  bind  it  firmly  to  the  framework.  Figs.  179  to  182,  inclu- 
sive, show  the  details  of  the  construction  of  one  of  these  jack- 
strips. 

The  hard-rubber  strip,  a,  forms  the  framework  for  each  strip 
of  twenty  jacks.     The  projections  at  its  ends  provide  for  attach- 


Fig.  181. — Bottom  View  of  Multiple  Jack-Strip. 

ment  to  the  switch-board.  In  this  strip  are  milled,  on  its  upper 
side,  the  transverse  grooves,  a1  a\  and  on  its  lower  side  similar 
grooves,  a*  a*;  these  being  best  seen  in  the  right-hand  portion  of 
Fig.  182. 

Perforations  are  drilled  from  the  front  of  the  strip,  one   per- 
foration to  each  pair  of  grooves,  having  its  axis  centrally  located 


1J1 


S3' 


d 

\ 


Fig.  182.— Details  of  Jack  Parts. 

and  parallel  with  respect  to  the  grooves.  A  small  portion  of  the 
hard  rubber  is  removed  from  between  the  grooves  so  as  to  leave 
a  rectangular  opening,  #6,  shown  in  Figs.  181  and  182,  through 
the  strip  connecting  the  two  grooves  at  those  ends  which  are 
nearer  the  front  of  the  jack.  In  the  grooves,  alt  upon  the  upper 
surface  of  the  rubber  strip  are  mounted  springs,  b  and  c.  The 
spring,  b,  is  the  longer  of  the  two,  so  that  its  curved  extremity  is 
presented  close  to  the  end  of  the  perforation  through  the  front 
portion  of  the  strip,  a.  The  springs  are  insulated  from  each 
other  by  a  strip  or  tongue,  d,  of  hard  rubber,  thin  and  flexible 
enough  not  to  impede  the  flection  of  the  two  springs.  In  the 


212  AMERICAN    TELEPHONE   PRACTICE. 

under  groove,  a"2,  is  mounted  another  spring,  b\  similar  to  spring, 
b,  and  of  equal  length. 

The  three  springs,  b,  c,  and  b\  are  firmly  secured  to  the  strip,  a, 
by  a  bolt,  e,  passing  through  them  and  the  body  of  strip,  a.  The 
bolt  is  insulated  from  the  springs,  bl  and  £,  by  rubber  washers 
and  bushings.  In  the  perforations,  a3,  in  front  of  the  strip,  are 
inserted  short  tubes,  /  of  brass.  Each  tube  or  thimble,  /  is  pro- 
vided with  a  shoulder,  which  bears  against  a  corresponding  ledge 
in  the  perforation,  a*,  so  as  to  prevent  the  tube  from  being 
thrust  backward  toward  the  rear  of  the  jack  by  the  insertion  of 
the  operators'  plugs.  The  thimble,  f,  is  provided  with  an  exten- 
sion, /',  to  afford  electrical  connection  with  it  from  the  rear  of 


Fig.  183.— Details  of  Plug. 

the  jack.  This  strip,/1,  extends  through  an  oblique  duct,/2 — 
shown  in  dotted  lines  in  Fig.  181 — and  thence  through  a  trans- 
verse slot  or  saw-cut,  a\  to  the  rear  of  the  strip. 

In  front  of  the  thimbles,/  in  the  perforations,  as,  are  placed 
the  test-rings,  very  short  tubes  of  brass,  g.  These  are  forced  into 
place  against  other  ledges  in  the  perforation.  The  ring,^,  is  also 
provided  with  an  extension,  g\  projecting  to  the  rear  of  the  strip 
of  spring-jacks.  These  extensions,  gl,  are  of  wire  and  pass 
through  another  duct,  g",  in  the  front  portion  of  the  strip,  a,  into 
a  saw-cut,  cf,  thence  to  the  rear  of  the  strip,  where  they  are  con- 
nected with  the  spring,  b. 

The  springs  in  these  jacks  are  of  hard  German  silver,  which  has 
been  found  the  most  desirable  material  for  this  and  similar 
purposes. 

The  detail  of  one  of  the  plugs  used  with  these  jacks  is  shown 
in  Fig.  183.  The  tip,  //,  of  brass  is  secured  by  the  rod,  7/1,  to 
the  block,  ff,  also  of  brass.  Insulated  from  the  tip  portion  by  a 
rubber  bushing  is  the  sleeve  contact,  h\  of  the  plug,  which  pro- 
jects rearwardly  and  forms  the  main  body  of  the  plug.  Over  this 
portion  is  slipped  a  shell,  //6,  of  hard  rubber  or  fiber,  which  forms 
a  handle  for  the  plug.  Between  the  tip  and  sleeve,  and  insulated 
from  each,  is  the  ring,  h\  which,  as  was  described  before,  is  for 
short-circuiting  the  springs,  b  and  bl,  when  the  plug  is  inserted  in 
the  jack. 

Screw  connectors,   k*   and   //6,   form    convenient   terminals   for 


THE   MULTIPLE   SWITCH-BOARD.  213 

attaching  the  strands  of  the  cord  to  the  tip  and  sleeve,  respect- 
ively, of  the  plug.  These  connectors  are  always  readily  acces- 
sible for  inspection  or  repair  by  the  removal  of  the  sleeve,  h\ 

This  brings  us  to  the  subject  of  flexible  cords  for  switch-board 
use.  The  matter  seems  at  first  thought  to  be  a  simple  one, .but 
much  thought  and  time  have  been  spent  in  perfecting  this  branch 
of  the  equipment.  Even  at  tliis  late  day  the  best  cord  is  not 
good  enough.  As  near  a  perfect  cord  as  has  up  to  the  present 
time  been  made  is  constructed  as  follows :  each  conductor  in  the 
cord  is  composed  of  strands  of  tinsel  and  a  few  fine  copper  wires, 
to  add  strength  and  conductivity.  Around  each  of  these  con- 
ductors are  wrapped  tightly,  in  opposite  directions,  two  layers  of 
floss  silk.  These  should  be  wrappings  and  not  braids,  as  a  wrap- 
ping serves  to  keep  any  broken  ends  of  the  tinsel  or  wire  down 
in  the  bunch  better  than  a  braid.  Around  these  wrappings  of 
silk  is  a  braid  of  linen,  which  adds  strength  to  the  conductor.  If 
the  cord  be  for  a  metallic  circuit,  the  two  conductors  thus  formed 
are  then  wrapped  with  two  more  layers  of  floss  silk,  and  the  whole 
is  then  encased  in  a  strong  spiral  wrapping  of  hard  spring  brass 
wire.  The  wire  in  this  spiral  is  stiff  enough  to  retain  its  shape, 
but  the  spiral  as  a  whole  is  so  flexible  as  to  allow  free  bending 
of  the  cord.  An  outer  covering  of  polished  cotton  is  tightly 
braided  over  the  spiral,  and  after  the  ends  are  tightly  anchored 
to  prevent  the  conductors  sliding  back  and  forth  in  the  spiral,  the 
cord  is  complete.  An  extra  layer  of  outside  linen  braid  is  often 
put  on  the  cord  for  a  distance  of  about  one  foot  back  of  the  plug, 
to  prevent  the  sharp  bending  or  kinking  of  the  cord  by  the 
operator  in  inserting  the  plugs  into  the  jacks. 

For  handling  very  large  exchanges  Mr.  Milo  G.  Kellogg  of 
Chicago  has  invented  a  number  of  divided  multiple-board 
systems,  some  of  which  will  probably  prove  important  factors  in 
the  telephone  industry  of  the  immediate  future.  In  these  he 
divides  the  lines  into  four  classes  and  the  switch-boards  into 
corresponding  divisions,  one  division  for  each  class  of  lines. 
Each  line  has  four  polarized  drops  and  four  answering  jacks,  one 
drop  and  jack  being  located  on  each  division  of  the  boards.  In 
addition  to  this  each  line  has  a  multiple  jack  on  each  section  of 
one  of  the  divisions,  the  arrangement  being  the  same  in  this  respect 
as  in  the  ordinary  multiple-board.  Two  of  the  polarized  drops 
are  of  opposite  polarity  and  are  connected  in  series  between  one 
side  of  the  line  and  ground.  The  other  two  are  also  of  opposite 
polarity  and  are  connected  in  series  between  the  other  side  of  the 
line  and  ground.  By  sending  a  current  of  ene  polarity  or  the 


214 


AMERICAN   TELEPHONE   PRACTICE, 


other  over  one  or  the  other  of   the   line   wires  and  ground,  a 
subscriber  may  thus  signal  any  division  of  the  exchange. 

In  operation,  if  a  subscriber  in  class  A  desires  to  call  one  in 
class  D  he  sends,  for  instance,  a  negative  current  over  the  test 
side  of  the  line,  which  operates  his  drop  at  division  D  of  the 
exchange.  As  the  called  subscriber  belongs  to  the  D  class  he 
will  have  a  multiple  jack  upon  each  section  of  the  D  division  of 
boards,  and  the  operator  will  answer  the  call  from  the  A  line,  by 
inserting  a  plug  in  the  answering  jack, — which  is  always  on  the 
same  section  as  the  drop, — and  complete  the  connection  by 


Fig.  184.— Kellogg  Divided  Multiple  Switch-Board. 

inserting  the  other  plug  of  the  pair  in  one  of  the  multiple  jacks 
of  the  D  subscriber  called  for.  The  calling  of  any  division  of 
the  exchange  is  done  by  an  arrangement  of  push-buttons  at  the 
subscriber's  station,  the  subscriber  always  pressing  the  button 
bearing  the  letter  of  the  class  to  which  the  subscriber  called 
belongs. 

In  Fig.  184  is  shown  diagrammatically  the  circuits  of  one  of 
the  Kellogg  divided  board  systems.  The  four  divisions  of  the 
board  are  represented  by  A,  B,  C,  and  D  ;  A1  A2,  B1  B2,  etc., 
representing  the  various  sections  of  board  in  each  division. 


THE   MULTIPLE   SWITCH-BOARD.  215 

Each  jack,  5,  is  composed  of  a  tip  contact,^,  a  sleeve  contact,/, 
a  test  contact,  d,  and  two  auxiliary  contacts,  a  and  b,  adapted  to 
be  pressed  into  engagement  with  each  other  and  with  the  test 
contact  by  an  insulated  portion  carried  on  the  connecting  plugs. 

L  and  L  are  two  subscribers'  lines.  Line  L  is  an  A  line,  and 
therefore  has  a  jack  on  each  section  of  division  A,  and  one  jack 
on  one  section  only  of  each  of  the  other  divisions.  Line  L 
belongs  to  class  C,  and  therefore  has  a  jack  on  each  section  of 
the  C  division  and  on  one  section  of  each  of  the  other  divisions. 

Line  cut-off  relays,  x  x,  are  provided,  one  for  each  line.  Each 
of  these  is  included  in  a  local  circuit  containing  the  common 
battery,  B ',  the  ground,  and  the  springs,  a  and  b,  of  each  jack 
belonging  to  its  particular  line.  These  relays  when  operated  cut 
off  both  taps  to  ground,  containing  the  line-drops,  w,  of  which 
there  are  four  for  each  line.  The  insertion  of  a  plug  into  a  jack, 
therefore,  by  pressing  together  contacts,  a  and  b,  operates  the 
relay,  x,  to  cut  off  all  the  drops  from  the  line.  It  also  connects 
the  test  contact,  d,  with  the  ground  at  G  through  the  spring  a  ; 
and  as  all  the  test  contacts,  d,  of  the  jacks  on  one  line  are  per- 
manently connected  together,  all  are  grounded.  This  ground- 
ing of  all  test  contacts  of  the  jacks  belonging  to  a  busy  line  is  to 
enable  an  operator  to  determine  the  condition  of  the  line.  The 
tip  side  of  her  cord  circuit  is  while  making  the  test  grounded 
through  an  impedance  coil  and  battery.  If  the  line  tested  is 
busy,  she  obtains  a  click  due  to  the  closed  circuit  from  the  tip  of 
her  plug  to  ground  through  the  test  contact  and  spring,  a.  If 
the  line  is  free  the  test  contact  is  not  grounded,  the  circuit 
remains  open,  and  no  click  is  obtained. 

The  advantage  claimed  for  this  division  of  multiple-boards  is 
the  enormous  saving  of  multiple  jacks.  The  limit  to  the  number 
of  subscribers  in  a  single  multiple  board  is  found  to  be  about 
6000,  a  greater  number  rendering  the  boards  so  cumbersome 
that  an  operator  cannot  reach  all  of  the  multiple  jacks  on  her 
section.  By  thus  dividing  the  exchange  into  four  divisions  it 
becomes  possible  to  place  as  many  as  24,000  subscribers  in 
a  single  exchange. 


CHAPTER   XX. 

TRANSFER   SYSTEMS. 

person  of  intelligence  can  visit  one  of  the  large  exchanges 
of  the  Bell  Company,  equipped  with  a  modern  multiple  switch- 
board, without  being  deeply  impressed  by  the  magnificence  of 
the  equipment,  the  perfection  of  the  system  in  its  entirety  and  in 
its  minutest  detail.  If  he  is  conversant  with  telephone  matters, 
he  must  also  be  impressed  by  the  fact  that  while  the  multiple 
switch-board  gives  the  subscriber  what  he  needs, — quick  reliable 
service, — it  gives  it  only  at  a  great  cost  to  the  operating  company. 
There  is  no  question  but  that  at  the  present  stage  of  telephonic 
development  the  multiple-board  systems  represent  the  highest 
type  of  central-office  equipment,  and  while  one  form  or  another  of 
them  are  in  use  in  nearly  all  the  really  large  exchanges  the  world 
over,  there  are  a  few  notable  exceptions.  The  success  of  some 
of  these,  coupled  with  the  enormous  expense  necessarily  entailed 
in  the  installation  of  large  multiple  boards,  leads  the  writer  to 
believe  that  the  coming  system,  while  it  may  in  some  degree 
embody  the  plan  of  multiple  jacks,  will  depend  for  its  action  on 
other  ideas  already  developed  to  a  considerable  extent. 

In  the  multiple  switch-board  the  cost  of  installation  increases 
approximately  as  the  square  of  the  number  of  the  subscribers. 
When  a  new  section  containing,  say,  200  spring-jacks,  annuncia- 
tors, and  other  apparatus,  is  added  to  an  exchange,  the  increase 
does  not  end  there.  Two  hundred  multiple  jacks  must  be  added 
also  to  each  of  the  sections  already  existing  in  the  exchange. 
This  is  clearly  an  objection  which  increases  in  seriousness  as  the 
exchange  grows.  A  multiple  board,  of  the  type  so  far  consid- 
ered, having  a  capacity  for  6000  lines,  would  probably  be  divided 
into  30  sections  of  200  lines  each.  On  each  section  would  be 
placed  6000  multiple  jacks,  plus  the  200  answering  jacks  belong- 
ing to  the  particular  200  lines  whose  annunciators  were  located 
at  that  section.  This  would  make  a  total  of  6200  spring-jacks  on 
each  section,  or  186,000  in  all.  The  first  cost  is,  therefore,  large; 
increasing  the  board  is  excessively  expensive,  and  cost  of  main- 
tenance is  necessarily  heavy. 

The  enormous  multiplying  of  jacks  in  the  multiple  system  is 
for  one  purpose — to  enable  each  operator  to  have  within  her 

216 


TRANSFER   SYSTEMS.  217 

reach  a  terminal  of  every  line  in  the  exchange,  to  the  end  that 
she  may  be  able  herself  to  complete  any  connection  called  for 
over  any  one  of  the  lines  under  her  immediate  supervision  : 
that  is,  that  she  may  be  able  to  answer  any  call  arising  at  her 
section,  and,  without  requiring  the  aid  of  any  other  operator, 
make  the  connection  called  forr  In  other  words,  in  a  6000  ex- 
change, 186,000  spring-jacks  would  be  used,  instead  of  only 
6000,  in  order  to  accomplish  this  result. 

Clearly,  if  multiple  jacks  are  not  used,  two  operators  or  more 
will  have  to  be  instrumental  in  making  a  connection  between 
two  subscribers  whose  lines  terminate  on  different  portions  of  the 
board.  It  would  seem  at  first  thought  that  this  would  be  a  con- 
siderable disadvantage,  and  would  result  in  a  slower  system.  On 
further  consideration,  however,  why  should  it  be  more  of  a  dis- 
advantage to  divide  the  labor  of  manipulating  a  switchboard  be- 
tween several  operators,  than  it  is  to  apportion  the  labor  of  mak- 
ing a  pair  of  shoes  or  any  other  manufactured  article  among  a 
large  number  of  operators,  as  is  now  done  in  all  large  factories? 

The  loss  of  time  required  by  one  operator  having  to  repeat  an 
order  to  another,  or,  as  is  the  case  in  some  systems,  of  the  sub- 
scriber having  to  repeat  his  own  order,  is  at  least  partially  com- 
pensated for  by  the  gain  in  simplicity  over  the  multiple  system. 

Systems  depending  for  their  operation  on  the  transfer  of  a 
connection  from  one  portion  of  a  board  to  another  are  termed 
transfer  systems,  and  one  of  the  most  successful  of  these  is  the  so- 
called  " express  system"  of  Messrs.  Sabin  &  Hampton  of  San 
Francisco.  This  system  has  been  used  for  several  years  in  San 
Francisco,  and  has,  according  to  reports,  demonstrated  its  capa- 
bility of  handling  with  success  an  exchange  having  over  6000 
subscribers. 

The  system  is  so  radically  different  from  anything  so  far  de- 
scribed that  its  consideration  in  detail  should  be  a  matter  of 
much  interest.  One  striking  feature  in  it  is  that  no  magneto- 
generators  are  used  at  the  subscriber's  station,  and  when  it  is  con- 
sidered that  approximately  one-third  of  the  cost  of  a  complete 
telephone  set  of  the  ordinary  type  is  in  the  magneto,  it  will  be 
seen  that  this  is  a  saving  of  considerable  moment.  The  doing 
away  of  the  magneto,  however,  is  not  an  essential  feature  of  the 
express  system,  but  is  one  of  the  advantages  incident  to  its  use. 

Briefly  stated,  the  underlying  ideas  of  the  express  system  are 
as  follows:  The  boards  are  divided  into  two  classes,  termed  for 
convenience  "  A  "  and  "  B."  Similarly,  the  operators  at  the  re- 
spective boards  are  termed  "A"  operators  and  "B"  operators. 


2i8  AMERICAN   TELEPHONE  PRACTICE. 

There  is  but  one  line  jack  for  each  line  in  the  exchange,  anct 
these  are  divided  into  groups  of  one  hundred  each  and  are  placed 
only  at  the  "  B  "  board.  At  the  "  A"  boards,  which  are  entirely 
removed  from  the  "  B  "  boards  (they  may  even  be  in  another 
exchange),  are  placed  plugs  which  form  the  terminals  of  other 
trunk  lines  leading  from  the  various  "B"  boards;  and  also  jacks 
forming  the  terminals  of  other  trunk  lines  leading  to  the  "B" 
boards. 

The  former  trunk  lines — that  is,  those  terminating  in  plugs  at 
the  "A"  boards — also  terminate  in  plugs  at  the  "B"  boards. 
These  are  termed  "A"  trunk  lines.  The  latter  trunks — that  is, 
those  terminating  in  jacks  on  the  "A"  boards — terminate  in 
plugs  on  the  "B"  boards,  and  are  termed  "B"  trunk  lines. 
When  a  call  is  received  it  attracts  the  attention  of  one  of  the 
"  B  "  operators  by  displaying  an  annunciator  in  the  ordinary  man- 
ner. The  "B"  operator  at  whose  board  the  call  is  received 
pays  no  further  attention  to  it  than  to  insert  one  of  the  plugs  of 
an  "A"  trunk  into  the  jack  of  that  line,  thus  transferring  the 
call  to  an  "  A  "  operator,  who  answers  it  with  a  listening  key  in 
the  ordinary  manner.  No  means  whatever  are  provided  for  a 
"  B"  operator  to  listen  in  on  a  subscriber's  circuit,  this  duty  be- 
ing confined  solely  to  the  "A"  operators. 

The  "A"  operator,  having  learned  that  the  subscriber  calling 
desires  to  be  connected  with  a  certain  other  subscriber,  conveys 
this  information,  by  means  of  a  special  order  wire,  to  the  "  B  " 
operator  at  whose  board  the  called-for  subscriber's  line  termi- 
nates. The  "  B  "  operator  then  tells  the  "  A  "  operator  what  "  B  " 
trunk  line  to  use,  and  the  "A"  operator  then  inserts  the  plug  of 
the  "A"  trunk  line  used  into  the  jack  of  the  "B"  trunk  line 
thus  designated.  This  brings  the  connection  as  far  as  the  board 
of  the  second  "  B  "  operator  ;  that  is,  the  "  B  "  operator  at  whose 
board  the  called-for  subscriber's  line  terminates.  This  operator, 
in  order  to  complete  the  connection,  simply  inserts  the  plug  of 
the  "  B  "  trunk  used  into  the  jack  of  the  called-for  subscriber's 
line,  and  presses  a  ringing  key  in  order  to  call  that  subscriber. 

It  will  be  seen  that  the  connection  has  really  been  handled  by 
three  different  operators,  but  that  the  first  of  these  operators  does 
no  more  than  to  insert  a  plug  into  a  jack,  giving  the  matter  no 
further  attention. 

All  signaling  between  the  subscribers  and  the  operators  and  be- 
tween the  various  operators,  whether  it  be  for  establishing  or 
clearing  out  a  connection,  is  entirely  automatic,  and  therefore  not 
dependent  upon  the  volition  of  the  parties  concerned. 


TRANSFER   SYSTEMS. 


2  19 


Fig.  185  shows  the  arrangement  of  the  circuits  at  the  sub- 
scriber's station,  and  also  the  arrangement  of  the  spring-jacks  and 
annunciators  on  the  "  B  "  boards.  One  side  of  the  line  wire  at 
the  subscriber's  station  is  normally  grounded  through  the  polar- 
ized ringer,  R.  This  means  that  calling  a  subscriber  from  the 
central  office  must  be  accomplished  over  one  limb  of  the  line 


C^fft^j^B  artery 


Fig.  185. — Subscribers'  Circuits — Express  System. 

wire  and  ground,  instead  of  over  a  metallic  circuit,  as  in  case  of 
talking  and  other  signaling  in  this  system.  The  other  circuits 
and  apparatus  at  the  subscriber's  station  are  of  the  ordinary  ar- 
rangement and  type,  the  only  difference  being  that  the  magneto- 
generator  is  omitted  entirely.  The  line  wires  of  each  subscriber 
terminate  in  two  springs,  a  and  b,  of  their  spring-jack,/".  These 
springs  normally  rest  on  two  anvils,  c  and  d,  one  of  which  con- 
nects through  an  annunciator,  e,  with  a  heavy  wire  leading  to  one 
pole  of  the  calling  battery,  and  the  other  of  which  leads  to  a 
similar  wire  connecting  with  the  other  pole  of  this  battery. 

This  annunciator,  e,  has  a  shutter  which  is  simply  lifted  by  the 
attraction  of  the  armature,  and  again  dropped  into  its  normal 
position  when  the  armature  is  released.  It  is,  therefore,  the 
simplest  type  of  self-restoring  drop.  The  circuit  of  the  call-bat- 


220  AMERICAN   TELEPHONE   PRACTICE. 

tery  is  normally  open  only  at  the  subscriber's  station.  It  is  auto- 
matically closed  through  the  receiver  and  secondary  winding  of 
the  induction  coil  at  the  subscriber's  station  whenever  the  sub- 
scriber removes  his  receiver  from  its  hook.  This  allows  enough 
current  from  the  calling  battery  to  pass  through  the  drop,  e,  to 
raise  its  shutter,  and  thus  attract  the  attention  of  the  "  B  "  opera- 
tor at  that  board. 

The  shutter  remains  raised  until  the  operator  inserts  the  plug 
of  one  of  the  "A  "  trunk  lines  in  order  to  transfer  the  call  to  the 
"  A  "  operator.  The  insertion  of  this  plug,  however,  lifts  the 
springs,  a  and  b,  from  the  anvils,  c  and  d,  thus  cutting  off  the 
battery  and  allowing  the  shutter  of  the  annunciator,  e,  to  drop  to 
its  normal  position,  and  the  "  B  "  operator  therefore  pays  no 
more  attention  to  it. 

A  single  battery  is  made  to  serve  for  actuating  the  signals  of 
every  line  in  the  exchange,  no  matter  how  great  their  number 
may  be.  Storage  cells  are  used  for  this  purpose,  ten  cells  being 
connected  in  series  so  as  to  give  a  pressure  of  about  twenty  volts. 
It  is  said  that  the  average  flow  of  current  from  this  battery  is 
about  one  and  one-half  ampere,  and  never  exceeds  two  amperes, 
in  the  San  Francisco  exchange  of  approximately  6000  subscribers. 
It  will  be  thus  seen  that  the  cost  of  maintenance  of  these  batteries 
is  trifling. 

Another  good  feature  of  this  arrangement  is  that,  should  a 
"  B  "  operator  by  mistake  withdraw  a  plug  from  a  jack  before  a 
subscriber  has  finished  talking, — that  is,  before  he  has  hung  up  his 
receiver, — she  will  be  at  once  notified  of  her  mistake  by  the  dis- 
play of  a  signal  belonging  to  that  line. 

In  Fig.  186  is  shown  a  simplified  diagram  of  the  express  sys- 
tem. At  the  bottom  and  top  of  this  figure  are  shown  the  sub- 
scribers' lines,  leading  in  each  case  from  the  subscriber's  telephone 
apparatus  to  the  drop  and  jack  at  the  central  office.  This  part  of 
the  apparatus  is  the  same  as  that  shown  in  Fig.  185,  but  in  this 
figure  the  details  of  the  local  circuit  at  the  subscribers'  stations 
have  been  omitted  for  the  sake  of  clearness. 

The  jacks  and  drops  belonging  to  these  lines  are,  as  has  already 
been  stated,  stationed  at  the  "  B  "  boards  of  the  exchange.  The 
subscriber's  indicator  battery  is  represented  at  vS  /  B  and  the 
indicators  at  /.  Leading  from  the  section  of  the  "  B  "  board 
shown  at  the  top  of  the  figure  is  a  trunk  line  leading  to  a  plug  on 
the  second  section  on  the  "  A  "  boards.  It  will  be  noticed  that 
this  trunk  line  terminates  in  a  plug  at  each  end,  and  is  termed 
the  "  A  "  trunk.  An  intermediate  jack  and  plug  are  shown  in  the 


TRANSFER   SYSTEMS. 


221 


circuit  on  this  "  A "  trunk,  but  these  at  present  need  not  be 
considered.  Suffice  it  to  say  that  the  plug  at  the  "  B  "  board  is 
connected  by  a  metallic  circuit  to  the  plug  at  the  second  section 
of  the  "  A  "  board.  Leading  from  a  certain  jack  on  the  second 
section  of  the  "  A  "  board  is  a  trunk  line  extending  to  a  plug  on 
another  section  of  the  "  B  "  Boards.  This  is  termed  a  "  B " 


Fig.  1 86. — Simplified  Diagram — Express  System. 

trunk.  Only  one  "  A  "  trunk  and  one  "  B  "  trunk  are  shown, 
but  it  must  be  remembered  that  a  number  of  "  A "  trunks 
lead  from  each  of  the  "  B "  boards  to  the  "  A "  boards,  and 
that  from  the  "  A "  boards  a  number  of  "  B  "  trunks  lead 
back  to  each  of  the  "  B  "  boards. 

When  a  subscriber,  as  for  instance  the  one  shown  at  the  top  of* 
the  figure,  removes   his  receiver  from  its  hook,  his  indicator,  /, 
is  displaced  automatically,  and   the   operator  at  the  particular 
"  B  "  board  at  which  this  indicator  is  located  extends  the  circuit 
of  his  line  to  one  of  the  "  A  "  boards  over  an  "  A"  trunk.    This 


222  AMERICAN   TELEPHONE  PRACTICE. 

she  does  by  inserting  the  plug  of  an  "  A  "  trunk  into  the  jack  of 
the  calling  subscriber.  The  operator  at  the  ''A  "  board,  having 
learned  the  desire  of  the  calling  subscriber,  extends  the  circuit 
to  that  subscriber's  line  by  means  of  the  "  B "  trunk,  still 
further  on  to  the  particular  "  B  "  board  at  which  the  jack  of  the 
called  subscriber  is  placed.  This  the  "A"  operator  does  by  in- 
serting the  plug  of  the  "  A  "  trunk  used  into  the  jack  of  a  "  B  " 
trunk  at  her  board.  The  "  B "  operator  at  whose  board  the 
calling  subscriber's  jack  is  placed  then  completes  the  connection 
between  the  two  subscribers  by  inserting  the  plug  of  the  "  B  " 
trunk  used  into  the  jack  of  the  calling  subscriber's  line.  The  two 
subscribers'  lines  are  shown  connected  in  Fig.  186  by  the  proc- 
ess and  over  the  circuits  just  described. 

In  order  to  facilitate  matters  it  is  evidently  necessary  that  a 
most  complete  and  elaborate  set  of  signals  must  be  .provided  be- 
tween operators.  The  first  in  this  series  of  signaling  operations 
is  to  notify  the  "  A  "  operator  that  her  attention  is  desired  on  a 
certain  "  A  "  trunk.  This  must  always  occur  just  after  the  "  B  " 
operator  has  inserted  one  of  the  "  A  "  trunk  plugs  into  the  jack 
of  the  calling  subscriber's  line.  It  will  be  noticed  in  Fig.  1 86  that 
the  relay,  R,  operating  signal  lamp,  L,  is  bridged  across  the  tip 
and  sleeve  strands  of  the  "  A  "  trunk  circuit,  and  this  bridge  may 
be  traced  from  the  tip  strand  through  the  balance  coil,  B  C, 
thence  through  the  clearing-out  indicator  battery,  C  O  B,  to  the 
battery  wire,  thence  through  the  coil  of  the  relay  at  the  '"  A  " 
board,  and  thence  to  the  sleeve  strand  of  the  "  A  "  trunk.  Re- 
membering that  the  calling  subscriber  at  the  top  of  the  figure 
has  removed  his  receiver  from  its  hook,  then  the  insertion  of  the 
plug  of  the  "  A  "  trunk  will  restore  the  line  drop,  /,  and  at  the 
same  time  will  close  the  circuit  from  the  clearing-out  indicator 
battery,  COB,  and  the  relay,  R,  through  the  subscriber's  line 
and  telephone  instrument.  This  will  operate  the  relay  and  cause 
it  to  close  the  circuit  of  the  signal  lamp,  Z,  thus  calling  the  atten- 
tion of  the  "  A  "  operator  to  the  fact  that  an  unanswered  call  is 
upon  the  trunk  line  to  which  that  lamp  belongs. 

The  apparatus  of  the  "  A  "  operator  is  shown  more  clearly  in 
Fig.  187.  The  sleeve  and  tip  strands  of  the  "  A  "  trunk  are  shown 
at  the  extreme  left  of  this  figure.  When  the  armature  of  the  re- 
lay, R,  is  attracted,  as  described  above,  the  circuit  from  the 
local  battery  and  white  lamp  is  completed  at  the  point,  b,  of  the 
relay.  It  will  be  noticed  that  this  local  circuit  extends  through 
two  of  the  springs,  e  and  f,  normally  closed,  on  the  listening  key 
of  the  "  A  "  operator,  and  also  through  a  pair  of  contacts,  /  and  c, 


TRANSFER   SYSTEMS.  223 

held  closed  by  the  weight  of  the  plug  of  the  "  A  "  trunk  in  its 
socket.  The  white  lamp  will  therefore  remain  lighted  until  one 
of  the  following  three  things  happens :  until  the  operator  lis- 
tens in,  which  causes  the  local  circuit  to  break  at  the  listening 
key ;  or  until  she  removes  the  plug  of  that  trunk  line  from  its 
socket,  which  would  break  the  local  circuit  at  the  point,  c;  or  until 
the  calling  subscriber  hangs  urj  his  receiver,  which  would  cause 
the  relay  to  let  go  of  its  armature,  and  thus  break  the  circuit 
at  point,  b.  The  white  lamp  remains  lighted,  therefore,  as  long 
as  the  call  on  its  "  A  "  trunk  is  unattended  to. 

The  first  act  of  the  "  A  "  operator  on  seeing  this  light  is  to 
throw  her  lever  corresponding  to  that  light  into  its  horizontal 
position,  thus  connecting  her  telephone  to  the  terminals  of  the 
"  A"  trunk  in  the  usual  manner.  This  enables  the  "  A  "  opera- 
tor to  communicate  with  the  calling  subscriber  in  the  ordinary 
manner.  It  should  be  noted  that  these  keys  on  the  "  A  "  opera- 
tor's boards  are  the  only  means  afforded  to  any  operators  for 
communicating  with  subscribers.  The  operation  of  this  key  breaks 
the  circuit  of  the  white  lamp  at  the  points,  e  f.  As  soon  as  the 
listening  key  is  thrown  again  into  its  normal  position  the  white 
lamp  is  again  lighted,  thus  calling  the  operator's  attention  to  the 
plug  to  be  used  in  making  the  connection. 

This  precaution  is  a  wise  one,  for  before  making  the  next  move 
in  the  connection,  the  "A"  operator  must  communicate  with 
the  "  B  "  operator  at  whose  board  the  called-for  subscriber's  line 
terminates.  The  "  A  "  operator  does  this  by  depressing  her  or- 
der-wire key,  shown  in  Fig.  186,  but  omitted  from  Fig.  187,  which 
act  connects  the  "  A "  operator's  telephone  directly  with  the 
telephone  set  of  the  outgoing  "  B"  operator.  The  "A"  opera- 
tor then  tells  the  "  B  "  operator  the  number  of  the  line  with 
which  connection  is  desired,  and  the  "  B  "  operator  in  return 
tells  the  "  A  "  operator  the  number  of  the  trunk  line  she  is  to 
use  in  making  the  connection. 

Herein  lies  one  of  the  greatest  defects  of  the  system.  That  is, 
the  necessary  waiting  by  the  "  A  "  operator  for  the  reply  of  the 
"  B"  operator.  This  is  a  loss  of  time  on  the  part  of  the  "A  " 
operator,  for  it  often  occurs  that  the  "  B  "  operator  may  have 
several  other  connections  under  way  which  she  could  not  well 
leave  in  order  to  reply  to  the  "  A  "  operator  by  designating  the 
number  of  "  B"  trunk  to  be  used. 

It  will  be  noticed  that  the  trunk  jacks  of  the  "  B  "  trunks  are 
in  reality  arranged  on  the  plan  of  the  multiple  board.  This  is 
for  the  purpose  of  placing  within  the  reach  of  every  "  A  "  opera« 


224 


AMERICAN   TELEPHONE   PRACTICE. 


tor  a  jack  belonging  to  every  "  B  "  trunk  line.  No  test  system, 
however,  is  required  on  these  jacks,  as  an  "  A  "  operator  always 
first  learns  from  an  outgoing  *'  B  "  operator  which  "  B"  trunk  to 
use,  and  of  course  a  "  B  "  operator  would  never  designate  any 
trunk  which  was  already  in  use,  or  "  busy." 

It  would  seem  that  a  "  busy  "  test,  or  preferably  a  visual  "  busy  " 
signal  for  the  multiple  jacks  on  the  "A"  boards  could  be  used 


CfiB. 

Fig.  187.— Table  of  "  A  "  Board. 

with  advantage.  This  would  enable  the  "  A  "  operator  to  at 
once  select  an  idle  "  B  "  trunk  and  at  the  same  time  inform  the 
proper  "B"  operator  of  the  connection  to  be  made  and  the 
trunk  line  plug  to  use  in  making  it.  For  instance,  the  "  A " 
operator  could  simply  say:  "  1504  on  10,"  meaning  by  the  first 
number  the  number  of  the  subscriber  called  for,  and  by  the 
second  number  the  trunk  line  to  be  used, 

We  have  now  carried  the  connection,  or  extended  the  circuit  of 
the  calling  subscriber's  line,  as  far  as  the  trunk  line  plug  at  the 
outgoing  "B"  board.  The  outgoing  "  B  "  operator  then  com- 
pletes the  connection  by  inserting  this  plug  into  the  jack  of  the 


TRANSFER    SYSTEMS.  225 

called  subscriber.  The  "  B  "  operator  then  depresses  her  ringing 
key,  shown  in  a  simplified  form  in  Fig.  186,  which  sends  calling  cur- 
rent from  the  generator,  G,  over  the  sleeve  strand  of  the  plug 
cord,  thence  to  line  and  to  ground  through  the  polarized  call-bell 
at  the  subscriber's  station. 

The  next  feature  to  considers  that  of  the  automatic  clearing- 
out  signals.  As  the  connection  between  the  two  subscribers  is 
made  by  three  operators,  it  is  evident  that  three  distinct  clear- 
ing-out signals  should  be  given,  one  at  each  of  the  boards  of  the 
operators  who  help  establish  the  connection.  Turning  again  to 
Fig.  187,  it  will  be  seen  that  the  raising  of  the  "A"  trunk  plug 
from  its  socket  changed  the  circuit  of  the  local  battery  from  the 
white  lamp  to  the  red  lamp,  by  moving  the  selecting  lever,  /,  from 
the  point,  c,  to  the  point,  d.  Remembering  now  that  as  long  as 
the  calling  subscriber's  receiver  is  off  its  hook,  the  circuit  from  the 
clearing  indicator  battery  is  closed  through  the  relay,  R,  at  the 
"  A "  board,  thus  attracting  its  armature.  As  soon,  there- 
fore, as  the  calling  subscriber  finishes  his  conversation,  he 
hangs  up  his  receiver,  and  thereby  breaks  the  circuit  through  the 
relay  at  the  "  A  "  board,  thus  closing  the  circuit  through  the  red 
lamp.  This  lamp  will  therefore  be  lighted  as  a  notification  to 
the  "  A  "  operator  that  disconnection  on  that  trunk  is  desired. 
The  replacing  of  the  plug  in  its  socket  opens  the  circuit  of  the 
red  lamp  at  the  point,  d,  thus  extinguishing  the  lamp.  This 
apparatus  at  the  "A"  board  is  very  ingenious,  and  deserves 
special  attention.  It  should  be  noticed  that  should  an  operator 
by  mistake  remove  one  of  the  "  A  "  plugs,  and  replace  it  in  its 
socket  before  the  subscriber  connected  had  hung  up  his  receiver, 
the  white  lamp  would  be  relighted,  thus  calling  the  attention  of 
the  operator  to  the  error. 

The  clearing-out  signal  is  given  to  the  incoming  "  B"  operator 
in  much  the  same  way  as  that  on  the  "  A  "  board,  the  clearing  indi- 
cator, or  relay,  on  the  "  A  "  trunk  of  the  "  B  "  board  being  wired 
in  multiple  with  the  relay  on  the  "  A  "  board. 

The  clearing-out  signal  on  the  outgoing  "  B  "  board  is  accom- 
plished by  much  more  complicated  means,  and  will  be  explained 
by  reference  to  Fig.  188.  In  this  figure  the  ringing  key,/,  is  shown 
in  more  detail  than  in  Fig.  186.  The  "  B  "  trunk  line  jack  on  the 
"  A  "  board  is  represented  by  a.  In  the  normal  position  of  the  key 
the  two  strands  of  the  "  B  "  trunk  are  connected  to  the  tip  and 
sleeve  of  the  corresponding  plug  on  the  outgoing  "  B  "  board. 
When,  however,  the  key  is  depressed,  the  sleeve  strand  of  the  cord 
is  connected  with  the  calling  generator,  the  other  terminal  of 


226 


AMERICAN    TELEPHONE   PRACTICE. 


which  is  grounded.  When  the  operator  depresses  this  ringing 
key,  a  secondary  pair  of  contacts,  i  il,  are  closed,  thus  actuating  the 
lower  magnet,  /,  of  the  compound  relay,  and  causing  it  to  attract 
its  armature,  /'.  When  the  operator  allows  the  key,/,  to  rise,  the 
armature,  /,  falls  back,  but  is  caught  by  the  hook,  m\  of  the  upper 
coil,  ;//,  of  the  relay.  The  hook,  m\  and  the  tip  of  the  armature 


a 


Fig.  188.— Table  of 

are  platinum-pointed,  and  their  con- 
tact causes  the  signal  lamp,  n,  to  be 
lighted.  This  lamp  remains  burn- 
ing until  the  called  subscriber  takes 
his  telephone  off  the  hook,  which  act 
closes  the  circuit  through  the  com- 
bined clearing-out  relay  and  signal,  c, 
in  exactly  the  same  manner  as  the  relays  on  the  incoming  "  A  " 
trunk  line  were  operated.  The  operation  of  this  relay  therefore 
closes  a  circuit  at  the  points,  f  c\  through  the  upper  magnet, 
m,  causing  it  to  raise  the  hook,  m\  and  allow  the  armature,  /',  to 
drop  back.  This  extinguishes  the  lamp,  n,  and  shows  the  opera- 
tor that  the  subscriber  has  responded.  The  armature,  c\  of  the 
relay,  c,  remains  attracted  until  the  called  subscriber  hangs  up  his 
receiver,  which  de-energizes  the  magnet,  c,  and  allows  the  signal 
carried  by  the  armature  to  resume  its  normal  position.  This  is 
the  clearing-out  signal  for  the  outgoing  "B"  operator,  and  she 
accordingly  pulls  out  the  plug. 

To  review  the  action  of  the  indicators  at  the  outgoing  "  B  " 
board,  the  releasing  of  the  key  for  transmitting  a  calling  signal  to 
the  subscriber  lights  the  lamp,  «,  and  shows  the  operator  that 
this  part  of  her  work  has  been  attended  to.  The  response  of  the 
subscriber  is  indicated  by  the  going  out  of  the  lamp,  and  by  the 
raising  of  the  signal,  k.  The  clearing-out  signal  is  given  by 
the  lowering  of  the  signal,  k. 


TRANSFER   SYSTEMS.  227 

We  have  now  traced  through  the  operation  of  all  the  signals 
between  the  subscribers  and  the  operators,  and  between  the 
operators  themselves,  which  were  necessary  to  establish  a  connec- 
tion between  two  subscribers ;  and  also  the  subsequent  signals 
between  the  subscribers  and  the  operators,  indicating  that  a  dis- 
connection is  desired.  The  striking  feature  of  all  this  elaborate 
system  of  signaling  is  that  each  signal  is  automatically  given  with- 
out the  volition  of  the  operator  or  subscriber,  inasmuch  as  it  is 
brought  about  by  some  action  necessary  in  the  actual  connection 
or  disconnection. 

In  order  to  reduce  the  work  of  the  operators  to  the  last  degree, 
two  phonographs  are  placed  in  connection  with  the  exchange,  one 
of  which  is  constantly  and  politely  repeating  the  sentence,  "  Busy. 
Please  call  again,"  while  the  other  repeats  with  equal  regularity, 
"  Subscriber  called  for  does  not  reply."  Each  of  these  phono- 
graphs speaks  to  a  transmitter  arranged  in  connection  with  an 
induction  coil  and  battery  in  the  ordinary  manner.  The  terminals 
of  the  secondary  of  the  induction  coil  of  the  "  busy  "  phonograph 
terminate  in  a  jack  on  each  section  of  the  "  A  "  boards.  In  like 
manner  the  "  does  not  reply  "  phonograph  is  connected  with  a 
jack  on  each  section  of  the  "  B  "  boards.  When,  therefore,  an 
"A  "  operator  learns  that  a  line  called  for  is  busy,  she  inserts  the 
plug  of  the  "  A  "  trunk  to  which  the  calling  subscriber  is  connected 
into  the  phonograph  jack,  and  the  familiar  but  disappointing  mes- 
sage, "  Busy.  Please  call  again,"  is  automatically  conveyed  to  the 
calling  subscriber. 

In  a  similar  manner  the  outgoing  "  B  "  operator  may  inform 
the  calling  subscriber  that  the  subscriber  called  for  does  not 
respond.  The  use  of  the  phonograph  for  this  purpose  may  seem 
at  first  thought  to  be  carrying  the  labor-saving  idea  to  an  extreme, 
but  it  enables  an  operator  to  attend  to  another  subscriber  while 
she  is  telling  the  first  subscriber  that  his  line  is  busy  or  that  his 
party  does  not  respond.  It  moreover  insures  that  the  wrath  of 
the  calling  party  will  produce  no  evil  effects  on  the  nerves  of  the 
operator,  which  at  busy  times  is  no  unimportant  consideration. 

The  writer  is  indebted  to  an  able  article  by  Mr.  George  P.  Low, 
in  the  Electrical  Journal,  for  much  information  concerning  this 
very  interesting  exchange  system. 

Fig.  189  is  a  schematic  representation  of  the  system  used  in 
the  larger  exchanges  of  the  Western  Telephone  Construction 
Company  of  Chicago.  This  system  has  proved  very  successful 
in  practice  for  exchanges  up  to  fifteen  hundred  subscribers, 
although  with  a  larger  number  certain  difficulties  are  met  in 


228 


AMERICAN    TELEPHONE  PRACTICE. 


the  disposal  of  the  transfer  plugs.  In  this  figure  I,  2,  3,  4,  etc., 
represent  different  sections  of  the  board,  each  section  having 
one  hundred  combined  drops  and  jacks  of  the  type  shown  in 
Figs.  158  and  159  together  with  a  complete  operator's  equipment. 
One  answering  plug,  A,  together  with  one  calling  plug,  B,  is  shown 
at  each  section.  These  are  connected  together  in  pairs  through 
clearing-out  drops,  O,  by  ordinary  flexible  cords  which  contain  the 


JO 


11 


$ 


5  k.ii 


LL  L 


ILL 


i_a 


Trrr 


Fig.  189. — General  Scheme  of  Western  Transfer  System. 

necessary  switching  apparatus  for  enabling  an  operator  to  listen 
in  and  to  ring  out  over  either  cord  as  desired.  Connected  with 
each  of  these  sets  of  plugs  is  a  trunk  line  to  which  is  connected 
at  every  third  section  a  transfer  plug,  C,  as  shown.  Thus,  a 
pair  of  plugs,  A  and  B,  shown  at  section  i,  is  connected  by 
means  of  a  trunk  line  to  a  transfer  plug  at  section  4,  another  at 
section  7,  another  at  section  10,  and  so  on.  A  careful  considera- 
tion of  this  figure  will  show  that  the  same  is  true  for  each  pair 
of  plugs,  A  and  B,  at  the  other  boards.  Fig.  190  shows  in 
greater  detail  one  pair  of  plugs,  A  and  B,  connected  by  a  trunk  line 
to  the  several  transfer  plugs  according  to  this  system.  The  plugs, 
A  and  B,  are  in  this  case  at  section  4,  while  the  transfer  plugs, 
C,  C,  C,  are  at  sections  I,  7,  and  10,  or  in  other  words,  at  every  third 
section  on  each  side  of  section  4.  The  form  of  circuit-changing 
lever  L  is  here  shown  for  convenience  only,  and  serves  to  illus- 


TRANSFER   SYSTEMS. 


229 


trate  the  principle,  but  not  the  actual  connections  in  this 
system.  By  throwing  this  lever  to  the  left,  its  two  springs  are 
connected  with  an  operator's  circuit  through  the  secondary  wind- 
ing of  the  'induction  coil,  as  shown,  while  when  the  lever  is 
thrown  to  the  right  the  terminals  of  the  generator,  G,  are  con- 
nected with  the  plug  circuit.  The  circuit-changer  actually  used 
in  this  cord  circuit  is  the  one  of  the  Western  Telephone  Con- 
struction Company,  described  in1  Chapter  XV.  The  operator 


immmm 


L-vwJ  L 

^yiiiJ 


t— VWV    [^ 

Ir^THJJ] 


Fig.  190. — Transfer  and  Instruction  Circuits. 

pulls  the  lever  forward  to  ring  the  called  subscriber,  pushes  it 
from  her  to  ring  the  calling  subscriber,  and  presses  it  downward 
to  listen  in. 

The  system  can  now  be  more  readily  understood  by  describ- 
ing its  operation.  If  a  subscriber  whose  line  terminates  in 
section  4  calls  up,  the  call  is  answered  by  the  operator  at  that 
board  by  inserting  one  of  her  plugs,  A,  the  insertion  of  this  plug 
restoring  the  shutter  mechanically.  The  operator  then  throws 
the  lever  Z,  to  the  left,  connecting  her  telephone  set,  E,  with 
the  line  of  the  subscriber  calling.  Having  learned  the  want 
of  the  subscriber,  who  we  will  say  is  1001,  the  operator  at  4 
depresses  the  key,  K,  which  connects  her  telephone  set  with  an 
instruction  circuit,  /,  terminating  in  the  telephone  set  of  the 
operator  at  section  10.  The  operator  at  section  4  is  thus  en- 
abled to  communicate  with  the  operator  at  section  10  over  this 
circuit,  and  the  former  informs  the  latter  of  the  number  desired 
and  of  the  particular  transfer  plug,  C,  she  is  to  use  in  making  this 
connection.  The  operator  at  section  10  then  takes  up  the  plug, 
C,  designated  and  inserts  it  into  the  jack  of  the  called  sub- 


230  AMERICAN    TELEPHONE  PRACTICE. 

scriber,  the  operator  at  section  4  meanwhile  holding  the  lever, 
Z,  of  the  particular  plugs  used,  into  the  ringing  position.  As 
soon  as  the  connection  is  completed  at  section  10  the  first 
operator  is  informed  of  the  fact  by  the  operation  of  a  buzzer 
placed  in  the  cord  circuit  so  that  she  knows  that  the  signal  has 
been  properly  transmitted  to  the  line  of  the  subscriber  1001. 
After  the  conversation  is  completed,  one  or  both  subscribers 
ring  off,  which  throws  the  clearing-out  drop,  O,  and  informs  the 
operator  at  section  4  that  a  disconnection  is  desired.  She  there- 
fore removes  her  answering  plug  and  places  it  in  the  socket,  in- 
forming the  operator  at  section  10  to  do  likewise.  It  will  be 
seen  that,  in  addition  to  the  transfer  lines  extending  from  the 
answering  and  calling  plugs  at  each  board  to  transfer  plugs  at 
each  third  board  therefrom,  a  system  of  instruction  circuits  is  also 
provided,  each  circuit  terminating  in  an  operator's  set  at  one 
board  and  connected  with  push-buttons  at  every  third  section 
therefrom,  so  that  an  operator  is  enabled  to  communicate  only 
with  those  operators  located  at  every  third  section  from  her  own 
board.  This  peculiar  arrangement  serves  several  advantageous 
purposes,  among  which  is  the  reduction  of  plugs  necessary  for  the 
successful  operation  of  the  board  and  also  the  reduction  of  the  num- 
ber of  operators  talking  over  any  one  instruction  circuit.  It  more- 
over enables  any  operator  to  reach,  by  means  of  her  own  calling 
plug  or  a  transfer  plug  handled  by  another  operator,  any  portion 
of  the  board.  For  instance,  if  a  subscriber  calling  on  section  4 
desires  connection  on  section  3  or  section  5,  the  operator  at 
section  4  will  complete  the  connection  herself  by  the  use  of  the 
calling  plug,  B,  as  she  can  readily  reach  any  jack  on  her  own 
section  or  on  that  at  her  right  or  left.  We  have  seen  how  a 
connection  is  made  between  section  4  and  some  section  at  which 
one  of  the  transfer  plugs  of  that  section  is  located.  If,  how- 
ever, the  subscriber  on  section  4  had  called  for  a  subscriber  at 
section  9  the  operator  at  4  would  have  signaled  the  operator 
at  10,  who  would  then  have  completed  the  connection,  using 
transfer  plug,  C,  with  her  left  hand.  If  the  called-for  sub- 
scriber had  been  upon  section  8,  operator  No.  4  would  have 
signaled  No.  7,  who  would  have  used  a  plug,  C,  at  her  section 
with  her  right  hand  to  complete  the  connection.  Ten  pairs  of 
calling  and  answering  plugs  are  furnished  for  each  section  of 
100  drops,  each  pair  being  connected  by  trunk  line  with  transfer 
plugs  distributed  through  the  system  as  already  described. 

A  system  of  lamp  signals  for  facilitating  the  w<5rk  upon  these 
boards  has  been  devised  and  used  in  many  of  the  later  exchanges. 


232  AMERICAN   TELEPHONE   PRACTICE. 

In  this  a  white  light  is  so  arranged  in  connection  with  the  night- 
alarm  circuit  as  to  be  illuminated,  upon  each  board,  whenever  a 
drop  is  thrown.  A  similar  lamp  in  series  with  this  is  also  ar- 
ranged to  be  displayed  on  the  chief  operator's  table,  thus  serving 
as  a  telltale  to  call  the  attention  of  the  chief  whenever  a 
drop  remains  unattended  on  any  section.  A  colored  lamp 
is  arranged  in  connection  with  each  set  of  transfer  plugs  and 
controlled  by  normally  open  contact  points  in  the  plug  seats 
of  the  transfer  plugs  and  normally  closed  contact  points  in  the 
plug  seats  of  the  answering  plugs.  Two  lamps  are  arranged 
in  series  in  each  circuit,  one  at  the  set  of  transfer  plugs  to  which 
it  belongs  and  the  other  at  the  set  of  answering  plugs  with  which 
these  transfer  plugs  communicate.  Whenever  an  operator 
raises  an  answering  plug  in  order  to  establish  a  connection, 
the  lamp  circuit  is  opened  at  that  point  by  the  operation  of  the 
contacts  in  the  plug  seat.  When  another  operator  removes  the 
transfer  plug  to  complete  the  connection  this  same  lamp  circuit 
is  closed  at  that  point  by  the  operation  of  the  contacts  in  the 
transfer  plug  seat.  The  circuit,  however,  still  remains  open  at  the 
answering  plug  seat.  When  a  calling-out  signal  comes,  and  the 
operator  removes  the  answering  plug  to  disestablish  the  con- 
nection, the  lamp  circuit  is  closed  at  its  only  open  point,  which 
lights  the  lamp  in  front  of  each  operator.  This  shows  the  trans- 
fer operator  that  a  disconnection  is  desired,  and  also  shows  the 
answering  operator  that  the  disconnection  has  not  yet  been 
made.  The  cycle  of  events  is  completed  when  the  transfer 
operator  removes  the  transfer  plug  and  replaces  it  in  its  seat, 
which  act  opens  the  lamp  circuit  at  that  point,  thus  putting  out 
both  lamps. 

The  switch-board  of  the  Delmarvia  Telephone  Company  at 
Wilmington,  Del.,  is  shown  in  Fig.  191.  This  board  embodies 
all  the  features  mentioned  above  and  is  undoubtedly  representa- 
tive of  the  best  exchanges  of  the  Western  Telephone  Con- 
struction Co. 

What  is  known  as  the  Cook-Beach  transfer  system  has  been  in 
long  use  among  some  of  the  Bell  exchanges  of  medium  size,  and 
the  large  switch-boards  now  manufactured  by  the  Sterling  Elec- 
tric Co.,  Chicago,  are  operated  upon  this  plan.  The  subscriber's 
lines  terminate  in  drops  and  jacks  on  the  various  sections  of  the 
board,  no  multiple  connection  whatever  being  used  between 
them.  A  set  of  transfer  jacks  is  also  provided  on  each 
section,  these  jacks  being  connected  by  trunk  lines  extending 
to  transfer  plugs  located  at  the  several  sections.  When  a 


TRANSFER   SYSTEMS.  233 

call  is  received  at  any  section,  the  operator  answers  it  by 
inserting  one  of  her  answering  plugs  into  the  corresponding 
jack.  Having  learned  the  number  of  the  subscriber  called  for, 
she  inserts  the  corresponding  connecting  plug  into  the  transfer 
jack  connected  by  a  trunk  line  with  a  plug  at  the  board  where 
the  line  of  the  subscriber  called  for  terminates.  She  then  com- 
municates with  the  operator  at  that  board,  who  picks  up  the 
transfer  plug  designated  and  inserts  it  into  the  jack  of  the  called 


Fig.  192. — 8oo-Line  Sterling  Switch-Board. 

subscriber.  The  connection  between  any  two  subscribers  is 
thus  made  complete  by  the  use  of  three  plugs.  This  style  of 
transfer  system  has  proven  its  adaptability  to  good  telephone 
service  by  long-continued  use  in  both  Bell  and  independent 
exchanges. 

A  board  embodying  this  plan  of  operation  is  shown  in  Fig. 
192.  The  line-drops  may  be  seen  in  the  upper  portion  of  the 
panel  of  each  section,  and  immediately  below  them  the  corre- 
sponding jacks.  The  drops  are  of  the  type  already  referred  to 
in  Chapter  XIV.,  and  are  provided  with  the  restoring  feature  by 
which  a  whole  vertical  row  of  shutters  are  restored  by  the  pres- 
sure upon  a  button  below  that  row.  Immediately  below  the  line 
jacks  are  shown  the  transfer  jacks,  these  being  of  such  a  nature 
that  when  a  plug  is  inserted  into  any  one  of  them,  a  signal  is 


234 


AMERICAN   TELEPHONE  PRACTICE. 


automatically  displayed  at  the  other  end  of  the  trunk  line  to 
which  it  belongs,  notifying  the  operator  at  that  section  that  a 
connection  is  desired  on  that  line.  The  listening  and  ringing 
keys  are  of  the  type  shown  in  Fig.  145.  Two  trunk  lines  are 
provided  from  each  loo-drop  section  of  this  board  to  every  other 
section,  this  number  being  found  to  give  an  ample  number  of 
trunk  lines  at  the  busiest  portions  of  the  day. 

The  larger  exchanges  equipped  by  the  American  Electric  Tele- 
phone Co.  use  a  transfer  system  somewhat  similar  to  that  of 
the  Cook-Beach  type,  composed  of  trunk  lines  extending  be- 
tween the  various  sections  of  the  board  and  terminating  in  jacks 
at  one  end  and  plugs  at  the  other.  Such  a  board  is  shown  in 
Fig.  193.  There  are  two  such  trunk  lines  extending  from  each 


Fig.  193. — 1200-Line  American  Switch-Board. 

ioo-drop  section  to  each  other  section,  thus  giving  four  trunk 
lines  between  each  two  sections — two  outgoing  and  two  in- 
coming from  any  position.  These  trunk  lines  terminate  in  jacks 
at  the  outgoing  ends,  and  plugs  at  the  incoming  ends.  Each 
operator  has,  besides  the  set  of  regular  listening  keys,  a  set  of 
instruction  keys,  one  for  each  of  the  other  operators,  the  de- 
pression of  any  one  of  which  connects  her  telephone  set  with 
the  set  of  another  operator  corresponding  with  that  key.  In 
this  way,  a  call  received  for  a  number  not  within  the  reach 
of  the  answering  operator  is  transmitted  to  the  operator  in 
whose  section  the  line  called  for  terminates,  being  given  by 
means  of  the  instruction  key  just  mentioned.  The  calling  line  is 
then  connected  by  a  pair  of  cords  and  plugs  to  the  jack  of  one  of 
the  two  transfer  lines  reaching  to  the  section  in  which  the  called- 
for  subscriber's  line  terminates.  The  establishment  of  this  con- 


TRANSFER   SYSTEMS.  235 

nection  causes  a  lamp  to  light  at  the  other  end  of  the  trunk  line 
and  shows  the  operator  there  which  of  the  two  lines  is  to  be 
used.  The  drops  used  are  the  same  as  those  illustrated  in  Figs. 
161,  162,  and  163,  the  shutter  being  restored  by  the  insertion  of 
the  plug,  and  again,  after  it  has  been  operated  as  a  clearing-out 
signal,  upon  the  withdrawal  oi{  the  plug  from  the  jack.  Calling 
is  accomplished  merely  by  pressing  the  calling  plug  to  its  fullest 
extent  into  the  jack  of  the  called  subscriber. 


CHAPTER  XXI. 

COMMON-BATTERY    SYSTEMS. 

IT  is  an  obvious  disadvantage  to  have  a  separate  source  of 
current  at  every  subscriber's  station  in  an  exchange ;  and  it  is 
not  to  be  wondered  at  that  many  efforts  have  been  made  to 
centralize  not  only  the  transmitter  batteries,  but  the  calling  cur- 
rent generators  as  well.  By  bringing  about  such  a  centralization 
of  the  sources  of  energy  many  desirable  results  are  attained. 
The  idle  capital  represented  by  the  local  batteries  and  the  calling 
generators  is  done  away  with — no  small  consideration  in  large 
exchanges,  because  the  magneto-generator  is  in  itself  the  most 
expensive  part  of  an  ordinary  telephone  set.  The  labor  of  visit- 
ing or  inspecting  the  subscribers'  apparatus  is  greatly  reduced ; 
that  necessary  to  repair  and  renew  batteries,  together  with  the 
•expense  of  material  for  such  renewal,  being  rendered  nil.  The 
subscribers'  instruments  are  made  neater  and  more  compact. 
The  electrical  efficiency  of  the  plant  is  greatly  increased  by 
having  a  few  large  units  in  operation  practically  all  of  the  time, 
instead  of  a  great  number  of  small  units  in  operation  but  a  small 
portion  of  the  time.  Lastly,  no  freezing  of  the  local  batteries 
occurs ;  there  is  no  spilling  of  the  acids  or  other  chemicals,  and 
no  corrosion  of  the  various  parts  by  fumes  therefrom. 

As  indirect  advantages  attained  in  the  most  modern  exchanges 
wherein  all  sources  of  energy  are  centralized,  may  be  mentioned 
the  fact  that  the  labor  on  the  part  of  the  subscriber  in  obtaining 
a  connection  or  a  disconnection  is  reduced  to  a  minimum,  and 
the  labor  on  the  part  of  the  operator  has  been  so  greatly  lessened 
as  to  enable  her  to  handle  with  success  about  twice  as  many 
subscribers  as  with  the  old  system.  Most  of  the  advantages 
enumerated  were  appreciated  by  telephone  men  long  ago,  and 
many  attempts  were  made  at  an  early  date  to  realize  them  in 
practice.  The  first  attempts  involved  a  return  to  first  principles, 
doing  away  with  the  induction  coil  and  placing  the  transmitters 
and  receivers  of  two  connected  subscribers  directly  in  series  in 
the  circuit  of  the  two  line  wires.  In  one  of  these,  made  in  1881, 
by  George  L.  Anders,  the  transmitter  batteries  were  placed  in  a 
loop  used  to  connect  the  circuit  of  two  line  wires.  In  this  the 

236 


COMMON-BATTERY  SYSTEMS.  237 

switch-board  was  of  the  old  cross-bar  type,  and,  while  it  used  no 
cord  circuits,  the  batteries  were  placed  in  series  in  the  connecting 
wire  corresponding  to  the  cord  circuit  in  later  exchanges. 

This  general  method,  as  applied  to  a  board  having  plugs  and 
flexible  cords,  is  illustrated  in  Fig.  194,  where  A  and  A  represent 


Fig.  194. — Series  Common-Battery  System — Grounded. 

two  subscribers'  stations  connected  at  the  central  office,  C,  by  a 
pair  of  plugs,  P  and  P',  having  a  battery,  B,  included  in  circuit 
between  them.  The  transmitter  and  receiver  of  each  subscriber's 
station  are  placed  in  series  in  the  line  wire,  and  each  transmitter 
when  operated  serves  to  vary  the  resistance  of  the  entire  circuit 
formed  by  the  two  connected  lines,  and  to  thereby  vary  the 
strength  of  the  current  flowing  from  the  battery,  B,  in  such 
manner  as  to  produce  the  desired  effects  in  the  receivers. 

In   Fig.   195    the    same    principle  of    operation    is   applied  to 


n 

T\ 


Fig-  195. — Series  Common-Battery  System — Metallic. 

metallic-circuit  lines,  two  of  which  are  shown  connected  at  the 
central  office,  C,  by  the  pair  of  metallic-circuit  plugs,  P  and  P '. 
In  both  of  these  cases,  in  which  the  battery  is  included  in  the 
cord  circuit  in  series  with  the  combined  circuit  of  the  two  lines, 
the  use  of  a  separate  battery  for  each  cord  circuit  is,  under 
ordinary  circumstances,  necessary.  This  is  always  true  of  the 
grounded  system  shown  in  Fig.  194,  and  is  also  true  of  the  me- 
tallic-circuit system  shown  in  Fig.  195,  unless  the  battery,  B,  is 
made  to  have  a  very  low  internal  resistance.  This  fact  was 
pointed  out  by  Mr.  Anthony  C.  White,  who,  in  1890,  showed  that 
it  was  possible  to  supply  all  of  the  cord  circuits  from  a  single 
battery  by  connecting  them  in  the  manner  shown  in  Fig.  196. 
This  involves  the  bunching  together  of  one  side  of  each  of  the 
cord  circuits,  the  battery  supplying  current  in  multiple  to  the 


238  AMERICAN   TELEPHONE   PRACTICE. 

various  pairs  of  lines  in  use  at  one  time.  This  figure  shows  four 
stations,  A,  A ',  A",  and  A'",  connected  in  pairs  by  two  cord  cir- 
cuits and  pairs  of  plugs.  Fluctuations  set  up  by  the  transmitter 
in  the  line  of  subscriber,  A,  will  circulate  in  the  combined  circuit 
of  the  lines  of  subscribers,  A  and  A'.  Similar  fluctuations  set  up 
by  the  transmitter  at  A"  will  flow  through  the  circuit  of  the 


Fig.  196. — Single  Battery  Series  System. 

lines,  A"  and  A'".  The  battery,  B,  and  the  wire,  a  and  a,  in 
which  it  is  included,  are  common  to  both  of  these  line  circuits, 
and  if  the  resistance  from  the  point,  a,  to  the  point,  a ',  through 
the  battery  is  considerable  in  amount,  a  part  of  the  fluctuations 
flowing  in  the  circuit  of  subscribers,  A  and  A',  will  be  shunted  by 
this  resistance  through  the  combined  circuits  of  the  subscribers, 
A"  and  A'".  If,  however,  the  resistance  ffom  the  point,  a,  to  the 
point,  a',  is  made  extremely  small,  practically  all  of  the  current 
changes  will  flow  through  the  battery  instead  of  being  shunted 
around  through  the  circuit  of  the  subscribers,  A"  and  A'",  owing 
to  the  comparatively  high  resistance  and  impedance  of  that  cir- 
cuit, with  its  included  instruments.  The  desired  reduction  in 
the  resistance  between  the  points,  a  and  a',  may  be  accomplished 
by  making  the  battery,  B,  of  extremely  low  resistance  and  by 
shortening  the  wire,  a.a',  which  is  common  to  all  of  the  circuits. 
The  former  result  is  accomplished  by  using  a  storage  battery  of 
rather  large  capacity,  and  the  latter  by  joining  the  various  cord 
circuits  directly  to  the  bus-bars  with  the  battery,  so  as  to  practi- 
cally eliminate  all  resistance  in  the  wire,  a  a'. 

The  common-battery  arrangement  shown  in  Fig.  197  is  one 
which  has  come  into  extensive  use  and  was  designed  by  Mr. 
John  S.  Stone  in  1892  and  1893.  In  this  figure,  A  and  A'  are, 
as  before,  two  subscribers'  stations  connected  by  metallic  circuit 
lines  with  the  central  office  at  C.  The  transmitter  and  receiver 
at  each  station  are  connected  in  series  in  the  line  circuit.  The 


COMMON-BA  T TER  Y  SYS TEMS. 


239 


battery,  B,  however,  is  connected  between  the  two  sides  of  the 
cord  circuit,  terminating  in  the  plugs,  P  and  P.  On  each  side 
of  the  battery  is  placed  an  impedance  coil,  /  and  /',  as  shown. 
The  action  in  this  case  is  as  follows :  the  current  from  the 
positive  pole  of  the  battery,  B,  flows  through  the  impedance 

coil,  /,  to  the  point,  a,  where  it  divides,  a  part  passing  through 

( 

A  C 

A  a .A 


Fig.  197. — Stone  Common-Battery  Arrangement. 

the  receiver  and  transmitter  of  each  of  the  subscribers'  stations. 
The  two  parts  of  the  current,  after  flowing  back  to  the  central 
office  through  the  opposite  sides  of  the  lines,  unite  at  the  point, 
a ',  and  flow  through  the  impedance  coil,  /',  to  the  negative  pole 
of  the  battery.  Inasmuch  as  the  impedance  coils  are  of  low 
ohmic  resistance,  they  offer  but  little  obstruction  to  the  passage 
of  this  current.  If  now  the  transmitter,  T,  at  station,  A,  is 
caused  to  lower  its  resistance,  the  difference  of  potential  between 
the  points,  a  and  a,  will  be  lowered.  This  will  result  in  a 


Fig.  198. — Stone  Common-Battery  Arrangement. 

diminution  in  the  current  flowing  in  the  line  of  subscriber,  A ' . 
On  the  other  hand,  if  the  resistance  of  the  transmitter,  T,  is 
raised,  the  difference  of  potential  between  a  and  a'  will  be 
raised,  thus  causing  a  greater  current  to  flow  through  the  instru- 
ment of  subscriber,  A ' .  Every  fluctuation  in  the  resistance  of 
the  transmitter,  caused  by  sounds  at  either  station,  will  thus 
cause  corresponding  fluctuations  in  the  current  flowing  through 
the  receiver  at  the  other  station,  thus  causing  them  to  reproduce 
the  sounds.  The  same  battery,  B,  is  used  to  supply  a  large 


240  AMERICAN   TELEPHONE   PRACTICE. 

number  of  cord  circuits,  the  arrangement  being  then  as  shown  in 
Fig.  198,  each  side  of  the  various  cord  circuits  being  con- 
nected to  the  poles  of  the  battery  through  impedance  coils,  as 
before.  The  fluctuations  set  up  in  the  circuit  of  the  two  sub- 
scribers, A  and  A',  while  perfectly  free  to  pass  through  these  two 
particular  lines,  cannot  find  a  path  to  any  other  lines,  as,  for  in- 
stance, those  of  subscribers,  A"  and  A'",  without  passing  through 
the  impedance  coils,  /  and  /',  and  also  /",  and  /".  It  is  said  that 
by  means  of  this  system  a  direct-current  generator  can  be  used 
in  place  of  the  battery,  B,  the  impedance  coils  serving  to 
effectually  weed  out  all  of  the  fluctuations  in  the  generator  cur- 
rent which  have  always  been  found  so  annoying  in  telephone 
work.  Notwithstanding  this,  however,  the  storage  battery  is 
always  used  in  practice  in  systems  embodying  these  principles. 

Early  in   1892   Mr.   Hammond  V.  Hayes  devised   the  method 
of  supplying  current  to  transmitter  batteries   shown   in   Fig.  199, 


Fig.  199. — Hayes  Common-Battery  Arrangement. 

this  having  come  into  very  extended  use  in  the  Bell  Companies, 
and  it  has  formed  the  basis  of  some  of  the  most  successful  common- 
battery  systems  in  the  world.  The  apparatus  at  the  subscribers' 
stations,  A  and  A',  is  arranged  as  in  all  of  the  preceding  sys- 
tems. At  the  central  office,  K  K'  are  repeating  coils  each 
having  two  windings,  k  and  k '.  The  two  windings  of  the  coil,  K, 
are  connected  together  at  the  point,  a,  which  is  connected  with 
the  positive  pole  of  the  battery,  B.  The  other  ends  of  these 
two  windings  are  connected  with  the  upper  contacts  of  the  plugs, 
Pand  P,  as  shown.  In  an  exactly  similar  manner  the  two 
windings  of  the  repeating  coil,  K ',  are  connected  together  at  the 
point,  a',  which  is  connected  with  the  negative  pole  of  the 
battery,  B,  the  other  two  ends  of  these  coils  being  connected 
with  the  lower  contacts  of  the  plugs,  P  and  P'.  By  this  arrange- 
ment  the  battery  is  included  in  a  bridge  conductor  between  the 
sides  of  the  circuit  formed  by  the  two  connected  lines,  and  one 
limb  of  each  line  includes  one  of  the  windings  of  one  of  the  re- 
peating coils.  The  current  from  the  battery,  B,  will,  when  the 
subscribers'  receivers  are  removed  from  their  hooks,  divide  at 


COMMON-BA  TTER  Y  S  Y STEMS. 


241 


the  point,  a,  and  pass  in  multiple  through  the  two  windings  of 
the  repeating  coil,  K,  thence  the  two  portions  of  the  current  will 
pass  through  the  transmitter  and  receiver  of  the  two  subscribers' 
stations  respectively  and  back  to  the  repeating  coil,  AT',  through 
the  windings  of  which  they  pass  to  the  negative  terminal  of  the 
battery.  ( 

Any  changes  in  the  current  in  either  circuit,  produced  by  the 
operation  of  one  of  the  transmitters,  will  act  inductively  through 
the  repeating  coils  upon  the  other  circuit,  causing  corresponding 
fluctuations  in  current  to  flow  through  that  circuit  and  actuate 
its  receiver.  Thus  when  the  subscriber  at  station,  A,  is  trans- 
mitting, the  windings,  k  k,  will  operate  as  a  primary  coil  of  an 
induction  coil  of  which  the  secondary  is  formed  by  windings, 
k  k ,  When  the  subscriber,  A,  is  transmitting,  this  action  is  re- 


i 


G  G 

Fig.  200. — Dean  Common-Battery  Arrangement. 

versed,  k  k  serving  as  a  primary  and  k  k  as  a  secondary  coil. 
The  transmitter  at  any  station  is  compelled  to  vary  the  resistance 
of  its  own  line  circuit  only,  and  in  this  way  some  of  the  advan- 
tages of  a  local  circuit  are  gained.  The  two  helices  of  each  repeat- 
ing coil  are,  under  ordinary  circumstances,  of  the  same  resistance 
and  number  of  turns,  and  wound  side  by  side  on  the  same  core. 
The  resistance  of  each  helix  is  usually  made  rather  low,  being  in 
the  neighborhood  of  five  ohms. 

All  of  the  systems  so  far  described  have  contained  the  sub- 
scriber's talking  apparatus  directly  in  series  in  the  line  wire.  Mr. 
J.  J.  Carty  is  responsible  for  the  broad  idea  of  supplying  current 
to  the  transmitter  of  the  subscriber's  station  over  the  two  sides  of 
a  metallic  line  circuit  in  parallel,  using  the  ground  as  return. 
This  method,  as  worked  out  by  Mr.  Carty,  has  been  improved 
upon  by  Mr.  W.  W.  Dean,  who  has  produced  an  extended  series 
of  inventions  embodying  this  feature.  One  of  them  is  shown, 
stripped  of  details,  in  Fig.  200,  in  which  A  and  A  are  two  sub- 
scribers' stations  and  C  the  central  office.  /  is  an  impedance 
coil  bridged  across  the  two  sides  of  the  cord  circuit  of  the  plugs, 


242  AMERICAN   TELEPHONE  PRACTICE. 

Pand  P.  The  center  point,  a,  of  this  coil  is  grounded  through 
the  talking  battery,  B.  The  receivers  at  the  subscribers'  stations 
are  connected  serially  with  the  secondary  coil,  s,  of  an  induction 
coil  in  the  metallic  circuit  formed  by  the  two  sides  of  the  line 
wire.  /'  is  an  impedance  coil  bridged  between  the  two  sides  of 
the  line  circuit  at  each  subscriber's  station,  the  center  point, /,  of 
this  coil  being  connected  with  one  side  of  a  primary  circuit  con- 
taining the  transmitter,  T,  and  the  primary  coil,/,  of  the  induc- 
tion coil.  The  opposite  side  of  this  primary  circuit  is  grounded 
at  the  point,  g.  Current  from  the  battery,  B,  flows  to  the  center 
point,  a,  of  the  impedance  coil,/,  in  the  cord  circuit;  thence 
through  the  two  sides  of  this  coil  in  multiple  to  the  points,  b  and 
Cj  on  the  opposite  sides  of  the  cord  circuit.  From  these  points 
the  current  flows  over  the  two  line  wires  in  multiple  to  the  points, 
</and  e,  from  which  they  flow  through  the  two  sides  of  the  impe- 
dance coil,  /',  at  the  subscriber's  station  to  the  point,  f,  where 
they  unite.  The  current  then  passes  to  the  primary  circuit  where 
it  again  divides,  part  passing  through  the  transmitter,  T,  and 
part  through  the  primary  coil,/.  It  reunites  at  the  point, g,  and 
passes  to  the  ground  and  back  to  the  battery,  B. 

Variations  in  the  resistance  of  the  transmitter  at  one  of  the 
stations  cause  more  or  less  of  the  supply  current  to  be  shunted 
through  the  primary,  /,  of  the  induction  coil,  and  these  varying 
currents  through  the  primary  induce  corresponding  currents  in 
the  secondary,  s,  placed  directly  in  the  line  circuit  with  the  re- 
ceiver. These  currents  flow  over  the  metallic  circuit  formed  by 
the  two  connected  lines,  and  are  prevented  from  flowing  through 
the  bridge  wires,  d  e  and  b  c,  by  the  presence  of  the  coils,  /',  and 
/,  contained  therein.  In  a  modification  of  this  scheme  Mr. 
Dean  uses  a  transmitter  having  two  variable-resistance  buttons, 
one  of  which  decreases  the  resistance  of  its  circuit  while  the  other 
increases  the  resistance  of  its  circuit.  One  of  these  buttons  is 
placed  in  each  of  the  branches  of  a  primary  circuit  such  as  is 
shown  at  the  subscriber's  station  in  Fig.  200,  each  side  of  the 
circuit  also  containing  the  primary  of  an  induction  coil.  These 
are  so  arranged  with  respect  to  the  secondary  that  an  increase  in 
current  flowing  through  one  of  them  produces  the  same  effect  on 
the  secondary  as  a  decrease  of  current  in  the  other,  and  there- 
fore the  effects  produced  by  the  two  variable-resistance  buttons 
of  the  transmitter  are  cumulative. 

The  use  of  secondary  batteries  at  the  subscribers'  stations 
supplied  by  some  source  of  current  either  at  the  central  office  or 
elsewhere,  has  been  occupying  the  minds  of  inventors  since  the 


COMMON'S  A  TTEK  Y  S  Y  STEMS. 


243 


very  early  days  of  telephony.  Storage  batteries  are  in  many 
respects  peculiarly  fitted  for  telephone  work.  Their  extremely 
-low  internal  resistance,  and  their  ability  to  maintain  a  constant 
E.  M.  F.  for  a  considerable  period,  are  obvious  advantages 
over  the  primary  battery.  Charles  E.  Buell  of  Plainfield,  N.  J., 
was,  in  1881,  the  pioneer  in  (this  line.  He  was  followed  by 
Stearns  in  1883,  Dyer  in  1888,  and  Dean,Stpne,  Scribner,  McBerty, 
and  others,  who  have  accomplished  much  in  this  line  of  work 
since  1893.  The  idea  of  Dyer  in  1888  was  to  charge  the  storage 
battery  from  the  ordinary  lighting  mains  of  a  city,  the  battery 
then  acting  in  a  local  circuit  containing  the  transmitter  and 
primary  of  an  induction  coil,  in  the  same  manner  as  when  a  pri- 
mary battery  is  used.  In  Fig.  201  is  shown  one  of  Stone's 


Fig.  201. — Storage  Cell  at  Subscriber's  Station. 

methods  which  involves  the  use  of  an  electrolytic  or  secondary 
cell  at  each  of  the  subscribers'  stations.  In  this,  advantage  is  taken 
of  the  fact  that  if  when  a  storage  cell  is  entirely  discharged,  a  charg- 
ing current  is  sent  through  it,  a  considerable  counter  E.  M.  F. 
is  set  up  by  the  cell.  The  battery,  B,  at  central  is  grounded 
through  an  impedance  coil, /,  its  other  terminal  being  connected 
to  the  center  point  of  a  divided  repeating  coil  bridged  across  the 
cord  circuit  of  theplugs,P/Yafterthe  manner  of  the  Hayes  system. 
Across  the  terminals  of  the  line  wires  at  each  subscriber's  station 
is  connected  the  secondary,  s,  of  an  induction  coil,  the  center 
point  of  which  is  grounded  through  a  secondary  cell,  5.  In 
circuit  with  this  cell  is  the  primary, /,  of  the  induction  coil  to- 
gether with  the  transmitter,  T.  The  current  from  the  battery, 
B,  passes  in  multiple  over  the  two  line  wires,  through  the  trans- 
mitter and  secondary  cell  in  multiple,  and  returns  by  ground. 
When  the  transmitter  is  operated  variations  in  current  in  the 
local  circuit  at  the  sub-station  are  produced,  and  these  act  in- 
ductively on  the  line  circuit  containing  the  receivers,  R,  by 
means  of  the  induction  coil.  If  the  cell,  5,  is  discharged  the 
transmitter  may  be  considered  as  acting  solely  by  means  of  the 


244  AMERICAN    TELEPHONE  PRACTICE. 

battery,  B,  the  counter  E.  M.  F.  of  the  electrolytic  cell  serving 
to  divert  a  considerable  portion  of  this  current  through  the 
transmitter,  and  thereby  accomplishing  the  same  result  as  if  the 
current  originated  in  the  cell,  5,  itself.  If,  l|owever,  the  cell,  S, 
is  fully  charged  then  the  transmitter  may  be  considered  as  work- 
ing upon  the  current  generated  by  it,  and  would  so  work  whether 
the  battery,  B,  were  in  circuit  or  not.  The  fact  that  the  second- 
ary cell  possesses  practically  no  resistance  and  no  inductance 
renders  it  especially  advantageous  for  this  work. 

The  use  of  storage  batteries  or  electrolytic  cells  at  subscribers* 
stations  makes  possible  a  full  realization  of  the  advantages  of  the 
induction  coil,  but  of  course  introduces  the  disadvantages  of 
having  fluid  cells  at  points  remote  from  the  central  office.  They 
have  been  used  in  some  cases  with  apparent  success,  but  their 
use  has  by  no  means  become  general. 

An  electrolytic  cell  acts  in  a  circuit  very  much  in  the  same 
manner  as  a  condenser,  and  systems  have  been  devised  in  which 
condensers  were  used  at  the  subscribers'  stations  in  place  of  the 
cells,  5,  shown  in  Fig.  201.  If  we  assume  these  cells  to  be  re- 
placed by  condensers,  the  other  arrangements  of  the  circuit 
being  left  as  shown,  current  from  the  battery  B  will  pass  over 
the  two  line  wires  in  multiple,  as  before,  and  to  ground  through 
the  transmitter,  T,  none  of  it  being  allowed  to  pass  through  the 
other  branch  of  the  primary  circuit,  by  virtue  of  the  condenser. 
When,  however,  the  transmitter  is  caused  to  vary  its  resistance, 
the  fluctuations  in  the  current  set  up  by  it  are  readily  trans- 
mitted through  the  condenser,  which  offers  to  them  practically 
no  impedance.  These  fluctuations  therefore  act  inductively 
upon  the  secondary  coil,  s,  of  the  induction  coil,  thus  causing 
corresponding  currents  to  flow  in  the  metallic  circuit  in  the 
ordinary  manner. 

Instead  of  using  a  storage  battery  at  the  subscriber's  station, 
Mr.  Dean  has  proposed  the  use  of  a  thermal  generator,  or 
thermopile,  to  produce  the  necessary  current.  As  is  well  known, 
If  the  alternate  junctions  of  a  thermopile  are  heated,  the  others 
remaining  cooler,  an  E.  M.  F.  will  be  set  up  by  the  pile.  An 
obvious  way  of  supplying  the  heat  is  to  wrap  the  junctures 
with  high-resistance  wire,  which  may  be  heated  by  the  passage 
of  a  current  through  it.  This  Mr.  Dean  does,  and  his  simplest 
arrangement  of  circuits  and  apparatus  is  represented  in  Fig.  202, 
in  which  the.  wires  of  a  telephone  line  are  shown  leading  to  the 
central  office  of  the  telephone  exchange.  The  telephone  re- 
ceiver, R,  and  the  secondary,  s,  of  the  induction  coil  are  placed  in 


COMMOAT-BA  TTER  Y  SYS 7 'EMS. 


245 


the  line  circuit,  as  in  the  instruments  now  in  use.  This  line  cir- 
cuit is  normally  open,  but  is  closed  by  the  hook-switch  when 
released  from  the  weight  of  the  receiver.  The  transmitter,  T, 
the  thermopile,  C,  and  the  primary,/,  of  the  induction  coil  are 
placed  in  series  in  the  local  circuit,  which  is  permanently  closed. 
The  resistance  coil,  r,  which  is  here  shown  in  proximity  to  the 
thermopile,  instead  of  being  wrapped  around  it,  is  in  a  circuit  in 
which  is  included  a  generator  (either  a  dynamo  or  a  battery).  It 
is  obvious  that  this  generator  may  be  placed  at  the  central  station, 
or  that  the  current  may  be  derived  from  the  street  mains  of  an 
ordinary  electric  light  circuit.  The  circuit  through  this  coil,  r,  is 


TELEPHONE  LINE  < 


Fig.  202. — Dean  Thermopile  Method. 

normally  broken  at  the  hook-switch.  When,  however,  the 
receiver  is  lifted  this  circuit  is  completed,  and  the  coil,  r,  becom- 
ing heated,  puts  the  thermopile  into  action.  The  thermopile 
therefore  generates  the  current  only  as  long  as  the  telephone  is 
in  use,  and  the  breaking  of  the  primary  circuit  becomes  unneces- 
sary. The  action  of  the  apparatus  in  talking  is  precisely  the 
same  as  if  a  chemical  battery  were  used. 

Mr.  Dean  has  worked  out  a  system  by  which  the  current  is 
applied  to  the  thermopile  over  the  two  wires  of  the  telephone 
circuit  in  multiple,  the  return  being  made  through  the  ground. 
Properly  arranged  retardation  coils  prevent  the  short-circuiting 
of  the  voice  currents,  but  allow  the  passage  of  the  comparatively 
steady  battery  or  dynamo  currents. 

All  of  the  principal  methods  for  supplying  current  from  a  cen- 
tral source  to  the  subscribers'  transmitters  have  now  been  pointed 
out ;  and  in  this  connection  it  may  be  well  to  show  how  in  large 
exchanges,  not  working  on  the  common-battery  principle  so  far 
as  subscribers  are  concerned,  a  single  battery  is  made  to  supply 
all  of  the  operators  transmitters.  The  old  method  was  to  use  a 


246 


AMERICAN   TELEPHONE   PRACTICE. 


separate  battery,  usually  two  or  three  gravity  cells,  on  each 
operator's  transmitter,  keeping  the  circuits  entirely  separate. 
Mr.  Carty,  however,  has  patented  the  method  shown  in  Fig. 
203,  which  is  largely  used,  and  which  gives  unqualified  satis- 
faction. 

In    this  B  and  B'  are  low-resistance  storage  cells,  preferably 
ced    in    multiple  so    as   to  make   their   joint    resistance    still 
lower.     The  primary  circuits  of  the  operators'  sets,  each  includ- 


t       t  t 

Fig.  203. — Carty  Multiple-Transmitter  Circuits. 

ing  a  transmitter,  7",  T',  and  T",  and  the  primary  winding,  p,  p ', 
and/",  of  their  respective  induction  coils,  are  connected  in  mul- 
tiple between  the  two  heavy  bus-bars  leading  from  the  termi- 
nals of  the  battery.  If  the  resistance  of  the  battery  is  very 
low,  and  the  bus-bars  are  heavy  and  short  enough,  no  cross- 
talk will  be  noticed  between  the  various  operators'  sets,  because 
the  resistance  of  the  battery  and  bus-bars  is  so  low  in  com- 
parison with  that  of  the  different  transmitter  circuits  that  the 
drop  of  potential  due  to  the  battery  resistance  w7ill  be  inappreci- 
able, and  therefore  a  fluctuation  in  the  resistance  of  one  of  the 
transmitters  will  cause  no  change  in  the  potential  at  the  battery 
terminals.  This  is  an  important  phenomenon  in  common- 
battery  work,  and  should  therefore  be  thoroughly  understood. 
It  may  be  more  readily  grasped  by  a  simple  application  of 
Ohm's  law. 

Let  ^t  represent  the  joint  resistance  of  the  transmitters  ;  that 
is,  of  the  path  from  one  bus-bar  through  the  several  transmitter 
circuits  in  multiple  to  the  other  bus-bar. 

Let  Rb  represent  the  resistance  of  the  battery  and  bus-bars, 
and  R  the  total  resistance  of  the  circuit. 

Let  E  represent  the  total  E.  M.  F.  of  the  battery,  and  e  the 
difference  of  potential  at  the  bus-bars. 


COMMON-BATTERY  SYSTEMS.  247 

Then  R  =  Rb  +  R  . 
By  Ohm's  law  the  current  is 

f~~''R  =^' 

Solving  for  e  we  obtain  < 

_  ERt  ERt 


R      '  Rb  +  Rt  ' 

For  a  condition  of  no  interference  between  the  various  trans- 
mitter circuits,  it  is  clear  that  variations  in  the  resistance  of  any 
of  the  transmitter  circuits  must  not  affect  the  difference  of 
potential  at  the  bus-bars.  In  other  words,  e  must  remain  con- 
stant. From  this  it  follows  that  the  fraction 


must  remain  constant,  since  E,  the  total  E.  M.  F.  of  the  battery, 
is  unchanging.  When  the  resistance  of  any  transmitter  is 
varied,  the  total  resistance,  R^  of  the  transmitter  circuits  will 
also  be  varied,  and  as  Rt  occurs  in  both  the  numerator  and  de- 
nominator of  the  fraction 


it  follows  that,  in  order  for  this  fraction  to  remain  constant,  the 
value  of  R^  must  be  'infinitely  small. 

Of  course  it  is  impossible  in  practice  to  obtain  a  battery  with 
no  internal  resistance,  but  a  single  1  5O-ampere-hour  cell  in  good 
condition  will  give  a  sufficiently  close  approximation  for  practi- 
cal purposes. 

In  actually  installing  a  system  of  this  kind  it  is  well  and  almost 
necessary  to  run  the  individual  wires  of  the  transmitter  circuits 
directly  to  the  terminals  of  the  storage  battery,  thus  practically 
eliminating  all  resistance  due  to  bus-bars.  The  writer  had  an 
intimate  acquaintance  with  a  case  where  serious  cross-talk 
occurred  under  the  following  conditions  :  The  battery  was  a 
single  2OO-ampere-hour  cell  of  the  American  type,  and  there  were 
ten  transmitters,  each  having  a  resistance  of  about  10  ohms,  the 
resistance  of  the  primary  coils  being  in  each  case  0.38  ohm.  The 
bus-bars  were  each  j  feet  long  and  each  composed  of  two  No.  6 
B.  &  S.  gauge  copper  wires  in  parallel.  Serious  cross-talk  existed 


248 


AMERICAN   TELEPHONE  PRACTICE. 


and  was  only  removed  when    the  bus-bars  were  made  of  ooo 
trolley  wire,  and  shortened  down  to  18  inches. 

Another  scheme  for  supplying  current  from  storage  batteries 
to  the  operators'  transmitters,  devised  by  Mr.  A.  R.  Hussey  of 
Chicago,  is  shown  in  Fig.  204.  In  this  B,  B,  B  are  storage  bat- 


7                        In                       In 

B 

H 

3 

F 

w- 

Si 

!U 
£|P 

Si 

M 

•J 

Fig.  204. — Hussey  Series-Transmitter  Circuit. 

teries,  of  one  or  two  cells,  connected  in  series  in  circuit  with  a 
dynamo,  D.  Looped  around  each  of  the  batteries  is  a  trans- 
mitter circuit  containing  a  transmitter,  T,  and  the  primary  wind- 
ing, /,  of  the  operator's  induction  coil.  This  works  well  and  is 
used  by  a  few  independent  exchanges. 

Mr.  Stone  has  devised  a  system  similar  to  this  by  which  no 
batteries  are  necessary,  the  current  being  utilized  directly  from 
a  dynamo.  The  circuits  are  the  same  as  those  of  Fig.  204,  with 
the  exception  that  condensers  replace  the  storage  cells,  B. 
Impedance  coils  are  also  placed  in  the  supply  circuit  on  each  side 
of  the  dynamo.  Any  changes  in  the  resistance  of  one  of  the 
transmitters  vary  the  potential  of  the  charge  of  the  condenser 
around  which  it  is  shunted,  and  therefore  cause  fluctuations  in 
the  current  through  the  primary,/.  The  current  actually  flowing 
through  that  coil  may  be  considered  as  the  resultant  of  the 
steady  current  from  the  dynamo  and  an  alternating  current  from 
the  condenser  superimposed  upon  it.  In  this  case  the  current 
remains  constant  in  the  supply  circuit  as  a  whole,  while  the  varia- 
tions in  current  set  up  by  the  transmitters  flow  freely  through 
the  corresponding  local  circuit  only.  The  fluctuations  of  the 
dynamo  current  are  with  this  arrangement  not  heard  at  all  in  the 
telephones.  The  dynamo  used  for  this  purpose  by  Mr.  Stone 
was  a  shunt-wound  machine  having  thirty-six  commutator  bars, 
running  at  a  speed  of  2200  revolutions  per  minute  and  generating 
12  volts.  The  impedance  coils  were  wound  to  have  a  joint  resist- 
ance of  67  ohms  and  were  provided  with  a  soft-iron  core,  common 


COMMON-BATTERY  SYSTEMS, 


249 


to  both  coils,  for  the  purpose  of  increasing  their  electromagnetic 
inertia.  The  capacity  of  the  condensers  was  about  6  microfarads. 

One  of  the  chief  advantages  of  common-battery  systems  is,  as 
has  already  been  pointed  out,  the  readiness  with  which  they  lend 
themselves  to  all  automatic  signaling  purposes.  The  methods  of 
supplying  current  to  the  subscribers'  transmitters  having  been 
described,  a  few  systems  embodying  these  methods  will  be  dis- 
cussed somewhat  in  detail  in  order  to  show  the  complete  working 
circuits  of  the  exchanges,  not  only  with  respect  to  the  means  for 
transmitting  speech  between  the  stations,  but  also  by  which  the 
various  signaling  operations  are  brought  about. 

The  system  of  Dean,  shown  in  simplified  form  in  Fig.  200,  for 


Fig.  205. — Complete  Circuits  of  Dean  System. 

supplying  current  from  the  central  station  over  the  two  sides  of 
the  line  wire  in  multiple,  is  illustrated  more  in  detail  in  Fig.  205, 
which  represents  the  circuits  as  they  would  be  in  actual  practice. 
The  impedance  coil,  /,  at  the  central  station  has  two  windings, 
7  and  8.  One  terminal  of  the  coil,  7,  is  connected  to  the  sleeve 
strand,  z,  while  one  terminal  of  the  coil,  8,  is  similarly  connected 
with  the  tip  strand,  i\  the  other  terminals  of  these  coils  are  con- 
nected together  at  the  point,  /,  which  forms  one  terminal  of  the 
common  battery,  B.  In  a  similar  manner  the  impedance  coil,  /', 
at  each  subscriber's  station  is  provided  with  two  windings,  I  and 
2,  connected  respectively  with  the  two  sides  of  the  line  circuits, 
a  and  a',  and  having  their  other  terminals  joined  at  the 
point,/.  The  iron  cores  of  these  impedance  coils  are  in  the  form 


250  AMERICAN   TELEPHONE  PRACTICE. 

of  flattened  rings,  in  order  that  a  complete  magnetic  circuit  may 
be  provided  to  increase  the  retardation  of  the  coils  as  much  as 
possible.  The  two  windings  on  each  coil  consist  of  about  3000 
turns  of  No.  22  silk-covered  wire.  As  a  result  of  this  construc- 
tion, the  coils  are  of  very  low  ohmic  resistance,  especially  when 
placed  in  parallel  as  they  are  with  respect  to  the  battery  currents  ; 
but  they  present  a  very  high  impedance  to  the  voice  currents 
flowing  in  the  metallic  circuit  formed  by  the  two  line  wires,  for  it 
is  evident  that  in  order  to  pass  from  one  side  of  the  circuit  to 
the  other  these  currents  would  necessarily  pass  through  the  two 
windings  of  the  impedance  coil  in  series.  The  currents  from  the 
battery,  B,  passing  through  the  two  windings  in  parallel,  produce 
no  magnetic  effect  upon  the  cores  of  the  impedance  coils,  and 
therefore  these  coils  are  in  a  condition  to  offer  a  maximum 
amount  of  retardation.  This  is  due  to  the  fact  that  a  mass  of 
iron  when  in  a  neutral  magnetic  state  is  more  susceptible  to  a  mag- 
netizing force  than  when  the  mass  is  polarized. 

At  the  sub-stations  the  supply  circuit,  after  being  united  at  the 
point,/",  again  divides  and  passes  through  the  two  halves  of  the 
primary  circuit  in  multiple  ;  but  in  this  case  two  primary  coils  are 
provided,  one  in  each  side  of  the  primary  circuit,  so  that  the 
changes  in  each  side  of  the  circuit  may  be  utilized  in  producing 
an  inductive  effect  upon  the  secondary  coil.  Thus  at  station,  A, 
the  circuit  divides  at  the  point,/",  one  part  passing  through  the 
side,/',  of  the  primary  circuit  containing  the  transmitter,  £,  and 
one  of  the  primary  coils,  represented  by  a  full  black  line  ;  and  the 
other  half  passing  through  the  branch,/2,  containing  the  resist- 
ance, g\  and  the  other  primary  coil,  represented  by  an  open  line. 
The  two  branches,/'  and  /2,  reunite  at  the  point,/3,  which  is 
grounded  through  the  resistance,^-2.  The  coil,  g' ,  has  about  the 
same  resistance  as  the  transmitter  in  its  state  of  rest,  so  that  the 
supply  current  will  divide  equally  between  the  two  halves  of  the 
primary  circuit,  and  therefore  normally  produce  no  magnetiza- 
tion of  the  core.  A  decrease  in  the  resistance  of  the  transmitter 
will  cause  a  greater  current  to  flow  through  side,  /',  of  the  pri- 
mary circuit  and  a  correspondingly  less  current  through  the  side, 
/2.  As  the  two  primary  coils  in  this  circuit  are  oppositely 
wound,  a  decrease  of  current  in  one  of  them  will  produce  the 
same  inductive  effect  on  the  secondary  as  an  increase  in  the  other, 
and  when  these  two  effects  take  place  simultaneously  in  the  pri- 
mary coils,  the  inductive  effects  upon  the  secondary  coil  are 
added.  An  increase  in  the  transmitter  resistance  will  in  the  same 
manner  induce  a  current  in  the  opposite  direction  in  the  secondary. 


COMMON-BATTERY  SYSTEMS.  251 

The  limbs,  a  and  a',  of  each  line  circuit  terminate  in  contacts 
on  the  hook-switch,  H,  so  that  when  the  hook  is  raised  the  con- 
nection is  completed  from  the  line  wires  through  the  telephone 
apparatus  already  described.  When  the  hook  is  down,  the  limb, 
a,  of  the  line  is  left  open  and  the  limb,  a,  is  closed  to  ground 
through  a  high-resistance  polarized  bell,  C. 

At  the  central  office  the  circuits  are  as  already  described,  with 
the  addition  of  the  line  annunciators,  AT  and  K',  and  the  clearing- 
out  or  supervisory  signals,  K"  and  K"'.  The  operator's  talking  set 
is  adapted  to  be  bridged  across  the  cord  circuit  by  the  listening 
key,  while  the  generator,  //,  may  be  connected  between  the 
ground  and  the  tip  of  the  calling  plug,  P,  by  the  key,  L. 

Assuming  the  apparatus  to  be  in  its  normal  position,  when 
subscriber,  A,  desires  a  connection  with  subscriber,  A',  he  raises 
his  receiver,  R,  from  its  hook,  H.  This  act  grounds  both  sides  of 
his  line  through  his  station  apparatus.  A  current  from  the  bat- 
tery, B,  thus  flows  through  the  drop,  K,  to  the  two  sides  of  the 
line  in  multiple,  by  virtue  of  the  fact  that  the  tip-  and  sleeve- 
springs  of  the  jack  rest  upon  the  common  anvil,/.  The  current 
flows  through  the  two  sides  of  the  subscriber's  circuit  in  multiple, 
and  to  ground,  and  is  of  sufficient  strength  to  cause  the  annun- 
ciator, K,  to  raise  its  target.  The  operator  seeing  the  signal  in- 
serts the  answering  plug,  P,  thus  cutting  off  the  circuit  through 
the  annunciator,  K,  and  allowing  its  target  to  assume  its  normal 
position. 

The  circuits  are  now  completed  from  the  battery,  B,  through 
the  two  halves  of  the  impedance  coil,  and  to  ground  at  the  sub- 
scriber's station,  as  already  described.  The  operator  then  bridges 
her  telephone  set,  71,  across  the  cord  circuit,  and  communicates 
with  the  subscriber.  Learning  that  subscriber,  A',  is  wanted, 
she  inserts  the  calling  plug,  P ',  into  the  jack  of  his  line  and  de- 
presses the  key,  Z,  which  connects  one  terminal  of  the  grounded 
generator,  g,  with  the  tip  strand  of  the  cord,  and  therefore  with 
the  side,  a,  of  the  line.  A  current  flows  from  the  generator 
to  ground  at  the  subscribers'  station,  and  operates  the  polarized 
bell,  C.  That  subscriber  then  removes  his  receiver  from  the 
hook,  and  the  two  converse  over  the  metallic  circuit  formed 
by  the  two  connected  lines. 

While  the  subscribers'  receivers  are  removed  from  their  hooks 
the  current  from  battery,  B,  flowing  through  the  sleeve  strand 
of  the  cord  circuit  energizes  the  magnets  of  the  clearing-out 
annunciators,  K"  and  K"'t  and  causes  them  to  lift  their  targets. 
As  soon  as  either  subscriber  hangs  up  his  receiver  this  current 


252 


AMERICAN    TELEPHONE  PRACTICE. 


ceases  to  flow,  because  the  line  wire,  a' ,  with  'which  the  sleeve 
strand  is  connected  is  opened  at  the  point,  d\  on  the  hook,  H. 
This  allows  the  target  of  the  annunciator,^''  or  K"',  to  fall,  show- 
ing that  that  subscriber  has  ceased  to  use  his  instrument. 

This  represents  perhaps  the  highest  development  attained  in 
any  of  the  methods  for  centralizing  all  energy  sources  of  tele- 
phone systems  wherein  the  current  for  the  transmitter  is  sup- 
plied over  the  two  sides  of  the  line  in  multiple.  Although  both 
calling  and  talking  currents  are  supplied  from  the  central  office, 

45- 


Lf 


Fig.  206. — Scribner  Common-Battery  System  for  Small  Exchanges. 

and  the  apparatus  at  the  sub-stations  is  greatly  simplified,  and 
all  signals  on  the  part  of  the  subscribers  are  automatically 
sent,  and  all  switch-board  drops,  both  line  and  clearing-out,  are 
self-restoring,  this  system  has  not  come  into  general  use,  probably 
on  account  of  the  still  greater  simplicity  of  the  Stone  and  the 
Hayes  systems. 

In  Fig.  206  is  shown  the  circuits  of  one  of  Scribner's  common- 
battery  systems  used  for  small  exchanges  based  on  the  Stone 
system  shown  in  Fig.  197.  This  is  representative  of  the  most 
modern  practice  in  this  line  of  work.  The  line  signal  is  automat- 
ically operated  by  the  removal  of  the  subscriber's  receiver  from 
its  hook,  and  is  effaced  by  the  insertion  of  a  plug  into  the  jack, 
which  act  opens  the  signal  circuit  at  the  jack.  Current  from  the 
battery,  /,  circulates  through  tke  impedance  coils,  5,  and  through 


COMMON-BATTERY  SYSTEMS.  253 

the  combined  circuit  of  two  connected  lines,  after  the  manner  of 
the  Stone  system,  already  described.  The  listening  and  ringing 
key  is  so  arranged  that  when  the  lever,  d,  is  moved  to  the  right  the 
wedge,  d ',  will  be  forced  between  the  springs,  e  and  e\  thus  connect- 
ing the  operator's  telephone  across  the  circuit  without  breaking  its 
continuity.  The  springs  and  the  wedge  arev  so  formed  that  the 
lever  will  remain  in  this  position  until  moved  by  the  operator. 
When  pressed  in  the  opposite  direction,  the  wedge  is  forced  be- 
tween the  springs,  e*  and  e3,  thus  connecting  the  generator  with 
the  calling  plug,  k '.  These  springs  are  so  formed  that  the  wedge 
will  be  forced  from  between  them  when  the  pressure  on  the  lever 
is  released. 

Arranged  in  one  side  of  the  cord  circuit  in  the  ordinary  man- 
ner are  the  supervisory  signals,  o  and  o' ',  these  signals  being  con- 
structed as  shown  in  Fig.  207,  which  also  gives  a  better  view  of 


Fig.  207.— Supervisory  Signals  for  Scribner  System. 

the  construction  of  the  listening  and  ringing  key.  The  indicators 
or  shutters,  a,  are  pivoted  at  their  edges  in  cavities  formed  in 
the  horizontal  key-table.  Each  shutter  is  provided  with  a  lug, 
a',  upon  which  bears  the  free  end  of  a  flat  spring,  b,  whose  other 
end  is  fixed  to  the  frame  of  a  tubular  magnet,  C,  arranged  under 
the  key-table.  This  spring  tends  to  bring  the  indicator  into  a 
horizontal  position,  as  shown  in  Fig.  207.  The  armature,  c\  of 
the  tubular  magnet,  c,  carries  an  arm,  c\  which,  when  the  arma- 
ture is  attracted,  is  thrown  against  the  spring,  b,  thus  pushing  it 
out  of  engagement  with  the  lug,  a',  on  the  shutter  and  allowing 
it  to  fall  from  view.  The  lever,  d,  of  the  listening  and  ringing 
key  is  connected  by  a  rod,/,  with  the  springs,  b,  of  the  annuncia- 
tors in  such  manner  that  when  the  lever  is  pressed  into  the 
listening  position,  as  shown  in  Fig.  206, -the  springs,  b,  will  be 


254  AMERICAN    TELEPHONE   PRACTICE. 

withdrawn  from  the  shutters,  thus  producing  the  same  effect  as 
if  the  magnets  were  energized,  and  allowing  the  shutters  to  drop 
out  of  sight. 

With  this  arrangement  the  keys  are  normally  left  in  their  lis- 
tening positions,  so  that  when  an  operator  inserts  an  answering 
plug,  kj  into  a  jack  in  response  to  a  call,  she  is  at  once  placed  in 
communication  with  the  subscriber.  Having  inserted  the  calling 
plug,  k ',  into  the  jack  of  the  called  subscriber,  she  moves  the  key 
to  the  ringing  position,  and  allows  it  to  spring  back  to  an  inter- 
mediate position  in  which  neither  the  telephone  nor  the  gen- 
erator is  connected  with  the  cord  circuit.  This  releases  the 
springs,  b,  from  the  influence  of  the  rod,/",  but  signal,  0,  will  not 
be  displayed  because  current  from  the  battery,  /,  is  passing 
through  the  line  of  the  calling  subscriber,  thus  energizing  its 
magnet  and  preventing  its  display.  As  the  called  subscriber  has 
not  yet  responded,  the  signal,  o',  will  be  displayed  because  suffi- 
cient current  cannot  pass  through  the  high-resistance  bell  of  the 
called  subscriber  to  energize  its  magnet.  As  soon,  however,  as 
the  called  subscriber  responds,  current  will  pass  through'  his  line 
and  the  signal,  o ',  will  be  effaced.  This  condition  will  be  main- 
tained until  one  or  both  of  the  subscribers  hang  up  their  receiv- 
ers, when  the  currents  through  the  respective  supervisory  signal 
magnets  will  be  cut  off,  their  armatures  will  be  released,  and  the 
shutters  will  be  displayed  by  being  forced  into  a  horizontal 
position.  The  operator  will  then  withdraw  the  plugs,  and  will 
move  the  lever  into  the  listening  position  in  anticipation  of  the 
next  call.  This  latter  act  will  cause  the  rod,/,  to  pull  the  springs, 
b,  out  of  engagement  with  the  shutters,  thus  allowing  them  to 
fall. 

In  Fig.  208  is  shown  a  common-battery  system,  as  applied  to 
a  multiple  switch-board,  embodying  most  of  the  latest  ideas  in 
telephone  exchange  work.  Two  subscribers'  lines,  L  and  D,  are 
shown  extending  from  the  subscribers'  stations,  A  and  B,  through 
the  spring-jacks,  J  and  /',  etc.,  on  the  various  sections  of  the 
switch-board.  For  clearness  the  two  jacks,  /,  are  shown  in 
separate  portions  of  the  diagram,  as  are  also  shown  the  two 
jacks,/';  but  it  must  be  remembered  that  the  jacks,/,  are  upon 
the  same  section  of  the  switch-board,  and  the  jacks,  /',  upon 
another  section.  Of  course,  in  a  large  exchange  a  far  greater 
number  of  jacks  than  two  would  be  connected  with  each  sub- 
scriber's line,  there  being  one  jack  for  each  line  upon  each  sec- 
tion. The  line  wires  of  each  metallic  circuit,  after  passing 
through  the  jacks,  pass  through  the  contacts,  8  and  9,  of  a  line 


COMMON-BA  T TER  Y  SYS1  'EMS. 


255 


cut-off  relay,  A,  the  circuit  between  them  being  completed 
through  a  battery,  7",  and  a  lamp  signal  relay,  B.  P  and  P' 
represent  a  pair  of  plugs  located  at  one  section  of  the  board,  it 
being  understood  that  this  pair  and  the  apparatus  shown  asso- 
ciated with  it  would  be  duplicated  many  times  at  each  section. 
Across  the  tip  and  sleeve  conductors,  a  and  b,  of  the  cord  circuit 
is  bridged  a  divided  repeating  coil  and  a  supply  battery,  E,  this 
arrangement  being  readily  recognized  as  that  of  the  Hayes  sys- 


Fig.  208. — Hayes  System  as  Applied  to  Multiple  Boards. 

tern.  The  other  apparatus  in  connection  with  the  cord  cir- 
cuit will  be  readily  understood  when  its  operation  is  described, 
it  being  only  necessary  to  state  that  the  relays,  O,  are  contained 
in  the  sleeve  strand  of  the  cord  circuit  and  serve  to  control  the 
circuits  of  the  relays,  N,  which  in  turn  serve  to  control  the 
circuits  through  the  supervisory  lamp  signals,  F. 

When  subscriber,  A,  removes  his  receiver  from  its  hook,  a 
current  from  the  battery,  T,  flows  through  the  line,  actuates  the 
line  signal  relay,  B,  and  causes  the  illumination  of  the  lamp,  C. 
The  operator  at  that  station  seeing  the  signal  inserts  the  plug,  P 
into  the  jack,  /,  thus  connecting  the  two  sides  of  the  line  with 
the  two  sides  of  the  cord  circuit.  This  allows  current  from  the 
battery,  E,  to  flow  through  the  circuit  of  the  subscriber's  line, 
and  this  current  causes  the  left-hand  relay,  (9,  to  attract  its  arma- 
ture and  thus  complete  the  circuit  through  the  relay  magnet,  N. 


256  AMERICAN   TELEPHONE   PRACTICE. 

The  insertion  of  the  plug  also  completes  the  circuit  from  the 
battery,  S,  to  the  conductor,  c,  and  contact,  c',  on  the  plug,  thence 
to  test-thimble,  5,  of  the  jack  and  by  wire,  6,  through  the  magnet 
of  the  line  cut-off  relay,  A,  to  ground.  The  current  flowing 
through  this  circuit  accomplishes  three  purposes :  first,  the  at- 
traction of  the  armature  of  the  relay,  N,  thus  breaking  the 
circuit  through  the  lamp  signal,  F;  second,  the  attraction  of  the 
double  armature  of  the  relay,  A,  thus  cutting  off  both  branches 
of  the  line  circuit  beyond  the  jacks  and  extinguishing  the  line 
signal,  C ;  and,  third,  the  raising  of  the  potential  of  all  of  the 
test-thimbles,  5,  connected  with  that  line  by  an  amount  equal  to 
the  drop  in  potential  through  the  relay  magnet,  A;  so  that  any 
operator  at  another  board  attempting  to  make  a  connection  with 
this  line  would  be  warned,  upon  touching  the  tip  of  her  plug  to 
the  test  thimble,  that  the  line  was  busy  by  a  click  in  her  head 
receiver.  Upon  learning  the  connection  desired,  the  operator 
applies  the  tip  of  the  plug,  P'y  to  the  jack  of  the  called  sub- 
scriber, and  if  his  line  is  free  she  will  hear  no  click,  because  the 
test-thimble,  5,  will  not  have  been  raised  to  a  higher  potential 
than  that  of  the  ground,  and  therefore  no  current  will  flow  from 
the  tip  of  the  plug  through  the  right-hand  upper  winding  of  the 
repeating  coil  and  to  ground  by  wire,  d.  Upon  the  insertion  of 
the  plug,  P't  into  the  jack  of  the  called  subscriber  the  current 
from  the  battery,  S,  will  pass  through  the  right-hand  cord  lamp, 
F,  through  the  rearward  sleeve  of  the  plug,  and  by  test-thimble 
to  the  line  cut-off  relay  of  the  called  subscriber's  line  to  ground. 
This  illuminates  the  lamp,  F,  and  operates  the  cut-off  relay,  as 
before.  The  lamp  remains  lighted  until  the  subscriber,  B, 
responds  to  the  call,  when,  upon  the  removal  of  his  receiver 
from  its  hook,  the  current  from  the  battery,  £,  is  allowed  to  flow 
through  his  line.  This  operates  the  right-hand  relay,  (9,  ener- 
gizes relay,  N,  and  thus  extinguishes  the  lamp,  F,  at  the  same 
time  allowing  enough  current  to  flow  through  the  magnet,  Nt  to 
serve  for  testing  purposes  and  to  hold  the  relay,  A,  closed. 

The  subscribers  now  converse  by  the  methods  already  dis- 
cussed, and  when  either  of  them  hangs  up  his  receiver  the  cir- 
cuit through  that  line  is  broken  at  the  condenser  and  the  cor- 
responding relay,  (9,  releases  its  armature.  This  de-energizes  the 
relay,  N,  and  causes  the  lamp  signal,  F,  to  become  lighted  as  a  sign 
for  disconnection.  Upon  removing  the  plugs  from  the  jacks  all 
apparatus  is  automatically  restored  to  its  normal  position. 

In  circuit  with  the  lamp,  F,  of  the  answering  cord  is  a  relay 
magnet,  M,  controlling  the  current  of  a  pilot  lamp,  //,  common  to 


COMMON-BA  TTER  Y  S  Y STEMS. 


257 


the  group  of  plugs  under  any  one  operator.  This  lamp  is  placed 
either  on  a  conspicuous  portion  of  that  switch-board  or  else  upon 
a  chief  operator's  switch-board,  and  serves  at  all  times  to  indicate 
to  the  chief  whether  or  not  that  particular  operator  is  properly 
attending  to  her  clearing-out  signals. 

This  particular  arrangement  pf  cord  circuit  relays  was  devised 
by  Mr.  H.  M.  Crane  of  Boston  ;  but  the  credit  for  the  system 
as  a  whole  must  be  shared  by -several  of  the  engineers  of  the 
Bell  and  of  the  Western  Electric  Company. 

In  Fig.  209  is  shown  one  of  Scribner's  systems  in  which  the 


Fig.  209. — Scribner  Multiple-Board  System. 

signaling  is  not  unlike  that  of  the  last  system  described,  but  in 
which  the  transmitter  supply  is  effected  substantially  by  the 
Stone  system  described  in  connection  with  Fig.  197.  In  this 
the  polarized  bells  at  the  subscribers'  stations,  A  and  A',  are  high- 
wound  in  order  to  avoid  the  necessity  for  the  use  of  a  condenser. 
These  bells  when  so  used  are  wound  to  a  resistance  of  5000 
ohms,  so  that  the  leakage  from  the  main  battery,  B,  is  not 
excessive  even  when  the  exchange  has  a  large  number  of  sub- 
scribers. The  reduction  in  resistance  brought  about  by  the 
raising  of  the  receiver  from  the  hook  causes  current  from  the 
battery,  B,  to  flow  through  the  lamp  signal  relay,  b,  and  through 
the  circuit  of  the  line  wire  back  to  ground  at  the  central  office. 


AMERICAN   TELEPHONE  PRACTICE. 

This  operates  the  lamp  signal  relay,  causing  it  to  attract  its 
armature,  thus  closing  the  circuit  of  the  battery,  D,  through  thie 
lamp  signal,  d,  and  causing  the  illumination  of  the  lamp  as  a 
signal  for  the  operator.  Upon  the  insertion  of  the  answering 
plug  the  same  chain  of  events  is  brought  about  as  in  the  sys- 
tem just  described.  The  cord  circuit  is  connected  with  the  line 
of  the  subscriber,  while  the  connection  of  the  sleeve,  m,  of  the 
plug  with  the  thimble,/",  of  the  jack  allows  current  to  flow  from 
the  battery,  D,  through  the  magnet  of  the  line  cut-off  relay,  e, 
which  operates  its  armature  to  cut  off  the  line  beyond  the  jacks. 
The  presence  of  the  connection  with  the  battery,  D,  raises  the 
potential  of  all  the  thimbles,  f,  of  that  line,  thus  causing  it  to 
test  busy  when  any  operator  at  another  board  applies  the  tip 
of  her  testing  plug  to  it.  The  battery,  B,  will  be  seen  to  be 
bridged  across  the  cord  circuit,  its  positive  terminal  being  con- 
nected with  the  conductor,  7,  of  the  cord,  through  the  impedance 
coil,  k,  and  wire,  10,  while  the  negative  terminal  is  connected  with 
the  conductor,  6,  through  the  ground,  the  wires,  8  and  9,  and  the 
two  impedance  coils,  k  and  /&2,  in  multiple. 

Inasmuch  as  the  subscriber  at  station,  A,  has  removed  his 
receiver  from  its  hook,  the  current  from  the  battery,  B,  flowing 
through  the  cord  circuit  operates  the  relay,  /,  thus  short-circuit- 
ing the  supervisory  signal,  o,  and  preventing  its  illumination. 
The  operator  communicates  with  subscriber,  A,  by  operating  her 
listening  key,  thus  connecting  her  telephone  set  across  the  cord 
circuit ;  and  at  the  same  time  disestablishing  the  continuity  of 
the  cord  conductor,  6,  except  through  the  two  windings,  k'  and  /£2, 
of  the  impedance  coil.  If  she  finds  that  the  line  of  subscriber, 
A',  is  not  engaged  she  inserts  the  corresponding  plug,  g\  of  the 
pair  into  the  jack  of  that  line,  and  operates  her  calling  key,  Ji. 
The  insertion  of  this  plug  operates  the  cut-off  relay,  e,  as  before. 
On  account  of  the  high-resistance  bell  at  the  called  subscriber's 
station  being  still  in  circuit,  sufficient  current  does  not  flow 
through  the  relay,  /',  to  cause  its  operation,  and  therefore  the 
lamp,  #',  is  illuminated  and  remains  so  until  subscriber,  A',  removes 
his  receiver  from  its  hook,  which  act  causes  a  low-resistance  path 
across  the  two  sides  of  the  line,  operates  relay,  /,  and  by  short- 
circuiting  the  lamp,  </,  extinguishes  it.  The  going  out  of  this 
lamp  informs  the  operator  that  subscriber,  A,  has  responded. 
The  two  subscribers  then  converse  by  the  means  outlined  in  the 
Stone  system  in  Fig.  197,  current  being  supplied  to  one  side,  7,  of 
the  cord  circuit  through  the  coil,  k,  of  the  impedance  coil,  and  to 
the  other  side,  6,  of  the  cord  circuit  through  the  impedance  coils, 


260  AMERICAN   TELEPHONE   PRACTICE. 

k'  and  /£2,  in  multiple,  these  having  their  upper  ends  connected 
by  means  of  the  contact  anvil  on  the  listening  key.  When 
either  subscriber  hangs  up  his  receiver  the  introduction  of 
the  high  resistance  of  his  bell  into  the  circuit  cuts  down  the  cur- 
rent from  battery,  B,  to  such  an  extent  that  the  corresponding 
supervisory  relay,  /  or  /',  releases  its  armature,  thus  breaking  the 
short  circuit  about  the  supervisory  signal,  o  or  o' ,  and  causing  its 
illumination.  The  illumination  of  both  of  these  signals  is  a 
sufficient  indication  for  the  operator  to  assume  that  the  con- 
nection is  no  longer  desired,  and  she  therefore  removes  both 
plugs,  restoring  all  of  the  apparatus  automatically  to  its  normal 
condition. 

The  test  in  this  system  is  performed  by  the  plug,  g',  in  the 
ordinary  manner.  If  the  test-thimbles,/",  of  the  line  tested  are 
raised  to  a  certain  potential  above  the  earth  by  the  insertion  of 
a  plug  into  a  jack  of  that  line  at  another  board,  the  current  will 
flow  from  the  thimble  through  the  tip  of  the  plug,  to  conductor,  6, 
of  the  cord  circuit,  and  to  ground  through  the  coil,  Jf.  This 
will  act  inductively  upon  the  coil,  k ',  so  that  a  current  will  flow 
through  it  and  the  operator's  telephone,  the  listening  key  of 
course  being  depressed. 

The  general  appearance  of  a  modern  multiple  switch-board, 
equipped  with  lamp  signals  controlled  by  cut-off  relays  in  a  man- 
ner already  described,  and  operating  on  the  common-battery  plan, 
is  shown  in  Fig.  210,  which  is  taken  from  a  photograph  of  the 
new  switch-board  recently  installed  by  the  Bell  Telephone  Co. 
of  Missouri  in  their  St.  Louis  exchanges.  This  board  consists  of 
19  sections,  with  three  operators'  positions  at  each  section.  It  is 
finished  in  mahogany,  and  is  about  six  feet  high,  four  feet  wide, 
with  an  over-all  length  of  nearly  115  feet.  It  is  at  present  wired 
for  4000  subscribers'  circuits,  and  is  capable  of  accommodating 
an  ultimate  number  of  5600  circuits.  Two  and  one-third  sec- 
tions are  reserved  for  incoming  trunks  from  the  various  branch 
exchanges,  located  in  the  different  districts  of  the  city. 

A  better  idea  of  the  construction  of  the  board  may  be  had 
from  Fig.  211.  In  addition  to  the  4000  multiple  calling  jacks 
shown  on  the  upper  panels  of  each  section  there  are  on  the  lower 
panels  of  the  switch-board,  as  illustrated  in  the  view,  260  answer- 
ing jacks,  appearing  only  in  that  particular  section  and  represent- 
ing the  set  of  subscribers'  lines  over  which  the  three  operators  at 
that  section  receive  their  calls. 

On  the  horizontal  keyboard,  below  the  jacks  just  referred  to,  is 
a  double  row  of  plugs,  the  rear  set  or  answering  plugs  being  those 


K 
c 


COMMON'S  A  T  TER  Y  SYS  TEMS. 


263 


first  used  in  the  answering  jacks  in  answering  a  call,  and  the  front 
set  being  used  for  testing  and  afterward  connecting  with  the  line 
of  a  subscriber  called  for,  at  the  multiple  jacks  above.  The 
listening  and  ringing  keys  may  be  seen  directly  in  front  of  the 
plugs. 

In  Fig.  212  a  rear  view  of  tjie  switch-board  is  shown,  giving  a 
clear  view  of  the  systematic  arrangement  of  the  line  and  relay 


Fig.  213. — Stromberg-Carlson  Common-Battery  Exchange  at  Battle 
Creek,  Mich. 

cables.  The  following  are  interesting  facts  in  relation  to  the 
wiring  of  this  exchange  :  there  are  five  million  six  thousand  feet 
of  wire  in  the  straightaway  cables,  and  nine  million  two  hundred 
and  eighteen  thousand  feet  of  wires  in  the  relays  and  other  coils. 
The  number  of  soldered  connections  between  the  terminals  of 
cables  on  the  main  distributing  board  and  the  operators'  switch- 
board is  not  less  than  one-half  million. 

In  Fig.  213  is  shown  a  view  of  a  common-battery  switch-board 
of   the    Stromberg-Carlson    Telephone     Manufacturing    Co.    of 


264  AMERICAN   TELEPHONE  PRACTICE. 

Chicago,  which  company  seems  to  be  doing  more  in  the  line 
of  common-battery  work  than  the  other  independent  manu- 
facturing concerns.  The  circuits  of  this  system  present  several 
points  of  interest,  and  the  writer  regrets  that  he  is  not  at  lib- 
erty to  publish  them,  having  been  requested  by  the  makers 
not  to  do  so. 


CHAPTER   XXII. 

HOUSE    SYSTEMS. 

TWO  general  plans  of  installing  interior  telephone  systems  for 
giving  service  between  the  various  departments  of  a  business 
establishment  may  be  followed  :  One  of  these  is  to  install  a 
switch-board  at  some  central  point  to  which  all  the  lines  radiate, 
and  at  which  they  are  connected  as  desired  by  an  operator.  In 
following  this  plan  the  switch-boards  and  instruments  used  may 
be  of  any  of  the  types  already  outlined  for  use  in  small  exchanges, 
The  second  plan  involves  the  use  of  what  is  called  an  intercom- 


nnsL 

^%1 

UOffiL 


COMMON  RETURN 


Fig.  214. — Circuits  of  Ordinary  House  Systems. 

municating  or  house  system,  in  which  the  instrument  at  each 
station  is  placed  on  a  separate  line,  the  line  belonging  to  each 
station  passing  through  all  of  the  other  stations.  By  means  of  a 
simple  switch  arranged  in  connection  with  each  telephone,  the 
party  at  any  station  may  at  will  connect  his  telephone  with  the 
line  belonging  to  any  other  station  and  call  up  the  party  at  that 
station  without  the  intervention  of  an  operator.  This  involves 
the  necessity  of  running  at  least  as  many  wires  as  there  are 
instruments  in  the  exchange  through  each  one  of  the  stations ; 
and  the  simplest  way  to  do  this  is  to  run  a  cable  having  the 
requisite  number  of  conduits  through  each  of  the  stations,  all  of 
the  conductors  in  the  cable  being  tapped  off  to  the  switch-con- 
tact points  on  each  telephone.  The  connections  for  a  system 
having  four  stations  is  shown  in  Fig.  214.  Each  of  the  telephone 

265 


266  AMERICAN   TELEPHONE   PRACTICE. 

sets  embraces  the  ordinary  talking  and  calling  apparatus  switched 
alternately  into  circuit  by  the  ordinary  form  of  hook-switch. 
These  instruments  differ  in  no  respect  from  the  ordinary  exchange 
telephone. 

Connected  with  one  of  the  binding  posts,  £,  of  each  instru- 
ment is  the  pivot  of  the  lever,  L,  which  lever  is  adapted  to  slide 
over  the  buttons,  I,  2,  3,  and  4,  arranged  in  the  arc  of  a  circle 
beneath.  Each  button  on  each  telephone  is  connected  with  a 
line  wire,  I,  2,  3,  or  4,  bearing  the  same  number  as  the  button. 
The  binding  post,  b',  on  each  instrument  is  connected  with  the 
common-return  wire  which  runs  through  the  same  cable  as  the 
line  wires.  During  the  idle  periods  of  each  instrument  the  lever 
is  kept  on  the  button  bearing  the  same  number  at  that  station. 
This  button  is  usually  called  the  home  button,  and  is  for  con- 
venience placed  at  the  extreme  left  of  the  row  of  buttons  on  each 
instrument.  The  apparatus  as  shown  represents  the  condition 
when  station  I  is  about  to  call  station  IV.  For  this  purpose  the 
party  at  station  I  has  moved  the  lever,  L,  from  its  home  button 
to  button  No.  4,  thus  connecting  the  instrument  at  station  I  with 
the  line  belonging  to  station  IV.  When  the  generator  at  station  IV 
is  operated,  the  current  flows  from  binding  post,  b,  to  the  common- 
return  wire  to  the  binding  post,  b',  at  station  IV,  thence  through 
the  generator  and  call-bell  at  that  station  to  binding  post,  b,  and  to 
lever,  Z,  whence  the  return  is  made  by  line  wire,  4,  to  the  lever, 
L,  and  the  binding  post,  b,  at  station  I.  When  the  receivers  at 
both  stations  are  raised  the  talking  apparatus  is  thrown  into  the 
circuit  over  which  the  calling  current  was  just  sent,  and  the 
parties  converse  over  the  common-return  wire  and  line  wire  No. 
4.  Had  station  IV  called  station  No.  I,  then  the  talking  and  ring- 
ing would  have  been  done  over  the  common-return  wire  and  line 
No.  I.  This  system  may  be  used  with  battery  call  instruments, 
such  as  is  shown  in  Fig.  90,  in  which  case  no  generators  or  polar- 
ized bells  will  be  required. 

The  great  drawback  to  the  system  of  wiring  shown  is,  however, 
that  the  lever  at  the  calling  station  must  always  be  moved  back 
to  the  home  button  when  a  conversation  is  finished.  If  this  is 
not  done  the  instrument  at  that  station  will  be  left  switched  upon 
the  wrong  line,  and  will  not  respond  to  a  call  sent  over  its  own 
line  from  another  party.  Moreover,  when  anyone  calls  a  party 
on  the  line  to  which  these  two  stations  are  left  connected,  both 
bells  will  ring,  thus  producing  much  confusion.  To  illustrate  this  : 
if  after  station  I  had  called  station  IV,  he  had  left  his  switch  lever, 
L,  in  the  position  shown,  station  II  could  not  call  station  I 


HOUSE    SYSTEMS. 


267 


because  the  instrument  at  station  I  would  no  longer  be  connected 
with  line  I.  Should  station  II  attempt  to  call  station  IV,  the  bells 
at  both  stations  I  and  IV  would  ring  because  both  of  those  in- 
struments are  connected  with  line  No.  4. 

Frequently  instead  of  using  a  rotary  switch  an  ordinary  plug 
and  cord  are  used  in  place  of  the  switch  lever,  while  the  buttons 
are  replaced  by  simple  spring-jacks  into  which  the  plug  may  be 
inserted.  In  Fig.  215  is  shown  such  a  system,  where  a  plug,  P, 


BATTERY  WIRE 


Fig.  215. — Common-Battery  House  System. 

in  each  case  takes  the  place  of  the  lever,  Z,  in  Fig.  214.  Ten 
line  wires  are  shown  in  this  figure  each  connected  with  ten  spring- 
jacks  on  each  of  the  telephone  instruments ;  the  wiring  of  but 
three  instruments  is  shown,  this  being  a  sufficient  number,  inas- 
much as  all  are  connected  to  the  circuits  in  the  same  manner. 
This  system  is  for  common-battery  work,  a  single  battery  located 
at  any  convenient  point  being  used  for  supplying  both  trans- 
mitter and  talking  current  to  all  of  the  stations.  This  battery 
is  connected  across  the  common-return  and  battery  wires,  which 
are  common  to  all  of  the  stations  and  which  are  placed  in  the 
same  cable  as  the  line  -wires.  Connected  between  the  common- 
return  wire  and  the  line  wire  bearing  the  same  number  as  its 
station  is  an  ordinary  vibrating  bell  the  circuit  through  which  is 
broken  when  the  receiver  is  removed  from  its  hook.  By  pressure 
upon  the  key,  k,  at  any  station,  circuit  may  be  completed  from 
the  common-return  wire  through  the  battery  to  the  plug,  P,  of 
that  station,  and  therefore  if  this  plug  is  inserted  into  the  jack 
belonging  to  any  other  station  the  pressure  upon  this  key  will 
cause  the  bell  to  sound  at  that  station.  In  this  way  a  call  may 
be  received  or  sent.  When  the  hook-switch  is  raised  the  trans- 
mitter of  a  station  is  connected  between  the  battery  wire  and  com- 


268  AMERICAN    TELEPHONE   PRACTICE. 

mon-return  wire,  so  that  all  of  the  transmitters  at  the  stations  in 
use  take  current  from  the  same  battery  in  multiple. 

In  order  to  reduce  cross-talk  between  two  or  more  pairs  of 
stations  which  happen  to  be  communicating  at  the  same  time, 
the  small  impedance  coils,  c  c,  are  placed  in  each  side  of  the 
transmitter  circuit  at  each  station.  These  coils  of  course  cut 
down  the  efficiency  of  the  transmission,  but  they  also  tend  to 
prevent  the  fluctuations  in  current  produced  in  any  transmitter 
from  backing  up  through  the  battery  wire  and  common-return 
wire  into  the  local  circuits  of  the  other  transmitters.  Fluctua- 
tions produced  in  the  local  circuit  of  any  transmitter  act  induct- 


Fig.  216.— Sixty-Point  Plug  Box  and  Desk  Set. 

ively  through  the  induction  coil,  /,  upon  the  talking  circuit 
containing  the  receiver,  the  circuit  being  completed  between  two 
stations  by  the  common-return  wire  and  the  wire  of  the  station 
that  has  been  called.  This  arrangement  necessitates  the 
removal  of  the  plug  when  through  talking,  as  otherwise  both  of 
the  stations  connected  would  be  rung  up  when  either  of  the 
stations  was  called. 

As  a  rule,  twenty  stations  are  considered  the  greatest  number 
that  may  satisfactorily  be  served  by  an  intercommunicating  sys- 
tem, and  when  a  greater  number  of  stations  is  to  be  installed  it  is 
better  to  use  a  central  office  provided  with  a  switch-board,  with 
an  dperator  in  attendance.  The  Stromberg-Carlson  Telephone 
Manufacturing  Companyof  Chicago  have,  however,  recently  devel- 
oped this  system  so  that  it  is  said  to  satisfactorily  serve  a  greater 


HOUSE   SYSTEMS.  269 

number  of  subscribers.  In  Fig.  216  is  shown  one  of  their  desk 
sets  in  connection  with  a  sixty-point  plug  board,  this  system 
being  arranged  for  intercommunication  between  sixty  stations, 
that  being  probably  the  largest  number  ever  successfully  served 
in  one  intercommunicating  system.  The  wiring  of  this  system 
is  so  arranged  that  the  difficulty  due  to  the  subscriber  leaving 
his  plug  in  the  wrong  spring-jack  is  practically  overcome. 

The  Holtzer-Cabot  Electric  Company  has  overcome  the  difficulty 
due  to  the  subscriber  calling  leaving  his  switch  lever  in  the  wrong 
position,  by  the  apparatus  shown  in  Fig.  217,  this  device  being 


Fig.  217. — Ness  Automatic  Switch. 

the  invention  of  Mr.  T.  VV.  Ness.  The  arrangement  is  such  that 
when  the  subscriber  hangs  up  his  receiver  the  switch  arm,  which 
is  under  the  influence  of  a  spring,  will  be  automatically  released 
and  will  fly  back  to  the  home  position  without  his  volition.  In 
the  figure  the  switch-restoring  mechanism  is  mounted  on  the 
inside  of  the  cover  of  the  box,  the  switch  lever  itself  being 
mounted  on  the  opposite  side.  The  lever,  L,  at  each  station, 
shown  in  diagram  in  Fig.  218,  is  adapted  to  slide  over  the 
buttons,  i,  2,  3,  and  4,  as  in  the  systems  already  described.  The 
curved  contact-piece,  D,  is  so  arranged  that  the  lever  will  not 
normally  engage  it,  but  by  pressure  upon  the  handle  of  the  lever 
it  may  be  brought  into  engagement  with  the  contact.  Referring 


270 


AMERICAN    TELEPHONE  PRACTICE. 


again  to  Fig.  217,  His  the  hook-switch  adapted  to  perform  the 
ordinary  functions  of  connecting  the  calling  and  talking  ap- 
paratus alternately  in  the  line  circuit.  The  switch  lever  is 
mounted  upon  the  shaft,  A,  which  may  be  seen  passing  through 
the  front  board  of  the  box  and  which  carries  a  ratchet-wheel,  £, 
of  hardened  steel.  A  coiled  spring  around  the  shaft  tends  to 
rotate  it  so  as  to  bring  the  lever  always  to  the  home  position. 
F  is  a  sliding  pawl  normally  held  in  its  lower  position  by  a 
coiled  spring  surrounding  it.  This  sliding  pawl  serves  to  hold 
the  lever,  L<  in  any  position  to  which  it  has  been  rotated,  by  the 
engagement  with  the  teeth  of  the  ratchet-wheel,  E.  Upon  the 


Fig.  218. — Diagram  of  Holtzer-Cabot  System. 


short  arm  of  the  hook-switch  is  pivoted  a  dog,  G,  adapted,  when 
the  receiver  is  placed  upon  the  hook,  to  engage  a  notch  in  the 
pawl,  F,  and  lift  it  out  of  engagement  with  the  ratchet-wheel. 
This  allows  the  spiral  spring  to  return  the  switch  lever  to  its 
right-hand  position  in  contact  with  the  home  button.  After 
raising  the  pawl  out  of  the  notch  on  the  ratchet-wheel  the  dog 
slips  out  of  the  notch  on  the  pawl,  thus  allowing  the  latter  to 
return  into  contact  with  the  ratchet-wheel,  in  order  to  be  ready 
for  the  next  use  of  the  telephone.  In  order,  however,  that  the 
pawl  may  not  engage  the  ratchet  before  the  lever,  Z,  has  fully 
returned  to  its  normal  position,  a  second  dog,  /,  is  provided, 
which  is  pressed  by  a  spring  so  as  to  occupy  a  position  under  the 
pin,/>,  carried  on  the  pawl,  thus  holding  it  out  of  engagement 
with  the  ratchet-wheel  until  the  rotation  of  the  lever  is  nearly 
completed.  At  this  point  a  cam  on  the  under  side  of  the  ratchet- 
wheel  pushes  the  dog,  y,  out  of  engagement  with  the  pin,  /,  and 
thus  allows  the  pawl  to  drop  into  position  against  the  ratchet- 
wheel.  It  will  be  seen  that  this  device  accomplishes  with  cer- 


HOUSE   SYSTEMS.  271 

tainty  what  the  memory  of  the  telephone  user  could  not  be  relied 
upon  to  do.  This  entire  mechanism  is  well  constructed,  all  of 
the  parts  subject  to  wear  being  of  hardened  steel.  The  diagram 
of  circuits  given  in  Fig.  218  shows  the  system  wired  for  four 
stations  operated  with  common  calling  battery,  and  with  local 
batteries  at  each  instrument  for  talking  purposes.  This  company 
also  manufactures  these  instruments  arranged  for  the  ordinary 
magneto  system,  in  which  case  the  wiring  may  be  substan- 
tially the  same  as  that  shown  in  Fig.  214,  but  without  its  dis- 
advantages. 


CHAPTER   XXIII. 

PROTECTIVE   DEVICES. 

THE  matter  of  protecting  telephone  apparatus  from  the  damag- 
ing effects  of  currents  other  than  those  which  properly  belong 
on  telephone  lines,  is  not  such  a  simple  one  as  might  be  at 
first  supposed.  The  "lightning  arresters"  found  on  nearly  all 
telephone  instruments  are  for  the  purpose  of  protecting  the 
instruments  only  from  what  may  be  called  high-tension 
currents,  such  as  those  produced  by  lightning.  The  usual 
form  of  this  arrester  is  shown  in  Fig.  219,  in  which  A  and  B 


Fig.  219. — Usual  Form  of  Telephone  Lightning  Arrester. 

represent  the  two  line  plates  carrying  two  binding  posts  and 
forming  the  terminals  of  the  instrument.  The  plate,  C,  is  con- 
nected with  the  ground.  These  three  plates  are  not  in  contact, 
the  idea  being  that  a  charge  of  lightning  will  jump  across  the  air 
gap  to  the  ground  plate  before  it  will  pass  through  the  high 
resistance  and  impedance  of  the  instrument  coils  within.  They 
do  some  good,  but  are  by  no  means  infallible,  as  lightning  has 
too  many  freaks  to  be  kept  out  by  any  such  simple  device. 
The  holes,  e,  f,  and  g,  are  for  the  reception  of  a  metallic 
plug,  which  if  placed  in  e  short-circuits  the  instrument,  and 
if  in  /  or  g  connects  either  one  side  or  the  other  of  the  line 
to  ground.  The  placing  of  the  plug  in  the  hole,  e,  affords  a  very 
efficient  means  of  protecting  an  instrument  during  a  storm,  but 
it  is  subject  to  the  very  grave  disadvantage  that  people  will  for- 
get to  remove  it  after  the  storm.  If  but  one  subscriber  is  served 
by  a  line,  no  one  is  hurt  but  himself,  but  if  it  is  a  party  line,  con- 


PROTECTIVE  DEVICES.  273 

siructed  on  the  bridging  principle,  the  insertion  of  such  a  plug 
causes  a  short-circuit  which  may  disable  the  whole  line.  Cases 
are  very  numerous  where  a  repair  man  has  had  to  drive  perhaps 
twenty  miles  in  order  to  tell  a  subscriber  to  remove  his  plug,  for, 
obviously,  the  subscriber  cannot  be  called  up  by  telephone. 

The  next  most  simple  means  consists  in  placing  in  the  tele- 
phone circuit  a  fuse-wire  of  very  small  current-carrying  capacity. 
These  wires  are  usually  mounted  upon  mica  strips,  which  may  be 
inserted  between  clips  forming  terminals  of  the  line  and  instru- 
ment wires.  Although  largely  used,  these  have  not  proved  at  all 
reliable,  but  they  often  save  an  instrument  when  the  one  shown 
in  Fig.  219  fails.  Considerable  difficulty  is  apparently  experi- 


Fig.  220. — American  Lightning  Arrester. 

enced  by  the  manufacturers  of  very  small  fuses  in  gauging  them 
to  blow  at  a  given  amperage.  It  is  frequently  found  that"  ^-am- 
pere "  fuses  carry  two  amperes  without  showing  any  signs  of 
blowing.  Again,  inasmuch  as  these  fuses  are  necessarily  very 
fine,  being  not  much  larger  than  a  hair,  it  is  a  very  easy  matter 
to  break  them,  thus  causing  an  open  circuit  the  location  of  which 
may  not  be  at  once  apparent.  In  the  case  of  a  high-tension  cur- 
rent these  fuses  usually  blow,  but  frequently  start  an  arc  across 
the  terminals,  which  does  the  damage  to  the  instrument  as  effect- 
ively as  if  the  line  wire  was  continuous.  An  instrument  is  some- 
times found  burned  out  with  its  fuse  still  intact,  although  this  is 
uncommon. 

Still  another  form  of  protector  consists  of  two  carbon  blocks 
held  apart  by  a  thin  disk  of  mica,  one  block  forming  the  terminal 
of  the  line,  the  other  being  grounded.  These  blocks  are  usually 
arranged  to  slip  in  pairs  between  rather  strong  springs,  so  that 
they  may  be  easily  removed  when  desired.  One  form  of  these, 
shown  in  Fig.  220,  represents  the  double-carbon  lightning  arrester 
of  the  American  Electric  Telephone  Company.  The  two  bind- 


274  AMERICAN   TELEPHONE  PRACTICE. 

ing  posts  at  the  top  of  the  figure  are  attached  to  the  two  branche^ 
of  the  line,  which  are  not  cut,  but  run  continuously  to  the  tele- 
phone instrument,  or  whatever  it  is  that  is  to  be  protected.  The 
third  binding  post  is  in  connection  with  the  ground  plate  upon 
which  the  two  lower  blocks  rest.  The  idea  in  this  is  that  a  cur- 
rent coming  in  over  the  line  will  jump  across  the  very  small 
space  between  the  carbons  and  pass  to  the  ground  without  harm- 
ing the  instruments. 

The  arrester  shown  in  Fig.  221   is  that   used   by  the  Western 


Fig.  221. — Western  Lightning  Arrester. 

Telephone  Construction  Company,  and  is  a  combination  of  the 
-carbon  and  the  fusible  arresters.  In  this  the  two  line  wires  of  a 
metallic  circuit  enter  the  two  binding  posts  at  the  right  of  the 
cut,  from  which  each  circuit  passes  through  the  fuse-wires 
mounted  on  the  mica  strips,  and  then  to  the  vertical  springs 
bearing  against  the  right-hand  carbon  block.  These  springs  are 
respectively  in  connection,  by  metallic  strips  underneath  the 
porcelain  block,  with  the  two  binding  posts  at  the  extreme  left 
of  the  figure.  The  single  binding  post  is  in  connection  with  the 
two  vertical  plates  holding  the  carbons,  and  is  grounded.  In  this 
the  fuse  is  supposed  to  blow  for  any  current  considered  too  great 
for  the  carrying  capacity  of  the  instrument,  while,  if  the  current 
is  of  a  high  enough  tension  to  form  an  arc,  it  will  jump  to  the 
ground  between  the  lightning  arrester  plates.  Some  advise  con- 
necting the  fuse  on  the  instrument  side  of  the  carbon  arrester  in- 
stead of  on  the  line  side,  but  this  is  not  best  in  most  cases  ;  for, 
if  there  is  a  cable  in  the  line,  and  a  cross  occurs  at  some  point 
beyond  it,  the  current  which  would  flow  through  the  telephone 
instrument  would,  owing  to  the  high  ohmic  resistance  of  the 
latter,  not  be  strong  enough  to  injure  the  cable  ;  but,  if  the  cur- 


PROTECTIVE   DEVICES. 


275 


rent  jumps  to  ground  through  the  carbon  arrester  a  practical 
short-circuit  is  formed  which  might  allow  a  very  heavy  current 
to  flow  through  the  cable  to  the  ground  and  thereby  damage  the 
conductor.  For  this  reason  it  is  better  to  have  the  fuse  on  the 
line  side  of  the  circuit. 

The  devices  so  far  described  have  been  designed  to  cut  off  all 
currents  from  the  apparatus  to  be  protected,  above  a  certain 
maximum  value.  It  frequently  happens,  however,  that  a  very 
small  current,  due  perhaps  to  a  cross  on  the  line,  will  not  be  suf- 
ficient to  blow  the  fuse,  and  will  yet,  by  reason  of  a  long-con- 
tinued flow,  store  up  enough  heat  in  a  switch-board  or  ringer  coil 


Fig.  222. — Hayes  Thermal  Arrester. 


to  char  the  insulation  or  burn  it  out  entirely.  These  currents 
are  the  telephone-exchange  manager's  worst  enemies,  and  are 
very  appropriately  termed  "  sneak  currents."  They  frequently 
pass  all  the  protective  devices  placed  in  the  circuit  to  arrest 
them,  without  producing  any  effect  whatever;  but,  on  reaching 
a  close  coil  of  wire,  they,  by  slow  degrees,  develop  enough  heat 
to  burn  out  the  coil,  or,  as  has  frequently  happened,  to  burn  up 
the  whole  exchange. 

A  device  to  afford  protection  against  such  currents  as  these, 
types  of  which  have  come  into  almost  universal  use  by  Bell  com- 
panies, is  termed  a  heat  coil,  and  was,  so  far  as  I  am  aware,  first 
introduced  by  Mr.  Hammond  V.  Hayes  of  the  Bell  Company  in 
Boston,  Mass.  This  device  is  illustrated  both  in  its  assembled 
state  and  in  its  various  details  in  Figs.  222,  223,  and  224.  In 
Fig.  223  are  shown  the  details  of  the  heat  coil  proper.  A  bobbin 
is  formed  of  the  two  disks  d,  and  dl,  in  the  thin  flat  space,  x, 
between  which  is  wound  about  10  ft.  of  No.  32  B.  &  S.  German- 
silver  wire.  On  the  side,  d\  is  carried  a  metallic  shoulder,  e,  and 
a  flange,  4,  forming  a  deep  groove,/,  between  them.  A  hole,  8, 


276 


AMERICAN   TELEPHONE   PRACTICE. 


is  formed  through  the  bobbin,  through  which  projects  a  hard- 
rubber  pin,  s.  The  pin,  s,  is  fixed  in  place  by  a  small  amount  of 
easily  fusible  solder,  which  normally  holds  it  in  the  position 
shown  in  the  left-hand  portion  of  Fig.  224.  One  terminal 
of  the  German-silver  wire  is  attached  to  the  flange,  e,  while  the 
other  terminal  is  attached  to  an  inclosing  ring,  d?2,  of  brass. 


Fig.  223. — Hayes  Heat  Coil. 

The  flange,  4,   and  the   ring,  d1,   are   thus   insulated  from  each 
other,  except  for  a  path  through  the  German-silver  wire,  w. 

Three  springs,  a,  b,  and  r,  are  mounted  as  shown,  upon  a  base 
plate,  A.  The  spring,  a,  which  forms  the  terminal  of  the  line,  I, 
is  slotted  at  a\  in  such  manner  as  to  receive  the  neck  or  groove, 
/",  of  the  heat  coil,  as  shown  in  Figs.  222  and  224.  When  in 
place,  the  spring,  bt  which  forms  the  terminal  of  the  instrument 
to  be  protected,  rests  against  the  ring,  d\  so  that  the  circuit  is 


Fig.  224. — Details  of  Hayes  Arrester. 

complete  from  the  line  wire,  I,  through  the  spring,  a,  flange,  4, 
German-silver  wire,  w,  ring,  ^2,  and  spring,  b,  to  wire,  2,  to  the 
instrument.  When  a  current  stronger  than  a  certain  predeter- 
mined value  passes  through  the  coil,  a  sufficient  amount  of  heat 
is  generated  in  the  wire,  w,  to  melt  the  solder.  This  allows 
the  spring,  c,  which  is  connected  by  the  wire,  3,  to  the 
ground,  to  push  the  pin,  s,  entirely  through  the  coil,  so  that  con- 
tact is  made  between  spring,  c,  and  flange,  4,  as  shown  in  the 
right-hand  cut  of  Fig.  224.  This  at  once  grounds  the  line  with- 
out leaving  any  air-gap  whatever  in  the  circuit,  as  in  the  previous 


PROTECTIVE   DEVICES. 


277 


arresters.  It  has  been  found  advisable,  however,  to  use  these 
heat  coils  in  connection  with  carbon  arresters,  and  also  with 
comparatively  heavy  fuses,  as  will  be  described  later. 


Fig.  225. — McBerty  Thermal  Arrester. 

Another  device  somewhat  similar  to  this,  but  adapted  to  open 
the  circuit  instead  of  connect  it  with  the  ground,  is  shown  in 
Figs.  225,  226,  and  227.  This  is  an  invention  of  Mr.  F.  R.  Mc- 


Fig.  226.— Parts  of  McBerty  Heat  Coil. 

Berty  of  the  Western  Electric  Company,  Chicago.  The  con- 
struction of  the  coil  itself  is  best  illustrated  in  Fig.  226,  in  which 
b  is  a  small  hollow  rivet  of  conducting  material,  upon  which  the 


Fig.  227. — Heat  Coil  of  McBerty  Arrester. 

coil,  a,  of  German-silver  wire  is  wound.  One  end  of  this  coil  is 
attached  to  the  shank  of  the  rivet,  as  shown,  and  the  other  end 
to  a  metallic  plug  or  button,/".  The  hook,  c,  is  soldered  into 


278  AMERICAN    TELEPHONE   PRACTICE. 

the  hollow  rivet,  b,  and  the  whole  is  inclosed  in  a  hard-rubber 
bushing,  d,  as  is  clearly  shown  in  Fig.  227.  A  hard-rubber  plug,  e, 
is  forced  into  the  cavity  in  the  bushing  after  the  rivet  has  been  put 
in  place,  and  this  serves  to  insulate  the  head  of  the  rivet  from 
the  metallic  plug,/".  A  forked  standard,  g,  is  mounted,  as  shown 
in  Fig.  225,  so  as  to  form  a  support  for  the  heat  coil  when  in 
place.  A  leaf-spring,  i,  insulated  from  the  support,  forms  one 
terminal  of  the  line  and  presses  firmly  against  the  plug,/".  The 
hook,  c,  of  the  heat  coil  is  engaged  by  a  spring,  /,  which  is  held 
thereby  under  a  considerable  tension.  This  spring  forms  the 
terminal  of  the  instrument  wire,  so  that  the  circuit  from  the  line 
passes  through  the  spring,  z,  the  plug,/,  the  coil,  a,  the  hook,  c, 
and  the  spring,  /,  to  the  instrument.  When  a  current  of  suffi- 


Fig.  228. — Combined  Carbon  and  Heat  Coil  Arrester. 

cient  strength  to  melt  the  solder  passes  through  the  coil,  the  plug, 
<:,  is  pulled  out  and  the  spring,  /,  is  thus  allowed  to  assume  its 
normal  position.  This  produces  a  wide  opening  in  the  line,  so 
as  to  prevent  arcing  across  the  gap. 

Heat  coils  may  be  so  adjusted  as  to  be  operated  by  extremely 
small  currents,  and  they  show  great  uniformity  in  their  opera- 
tion. By  varying  the  length  of  the  resistance  wire  or  its  size, 
they  may  be  made  to  respond  to  a  given  current  in  almost  any 
length  of  time  desired.  The  times  of  operation  for  coils  con- 
structed  in  the  same  manner  will  seldom  vary  over  I  per  cent,  from 
each  other.  These  coils  are  usually  adjusted  to  act  when  sub- 
jected to  a  current  of  one-quarter  ampere  for  thirty  seconds  ; 
they  are,  however,  sometimes  adjusted  for  currents  as  low  as  .15 
ampere.  In  later  coils  about  30  ins.  of  No.  31  B.  &  S.  German- 
silver  wire  is  used.  Heat  coils  of  types  similar  to  these  are  now 
built  in  several  different  forms  and  are  generally  combined  with 
carbon  arresters.  In  protecting  switch-boards,  it  is  of  the  utmost 
importance  that  arresters  be  provided  for  each  side  of  each  drop, 
and,  as  economy  of  space  is  a  very  important  item  in  telephone 


PROTECTIVE  DEVICES. 


279 


exchanges,  it  becomes  necessary  to  arrange  them  in  as  compact 
a  manner  as  possible.  Fig.  228  shows  combined  carbon  arresters 
and  heat  coils  mounted  on  long  strips  of  iron  for  this  purpose. 
The  principles  of  operation  are  the  same  as  those  already  de- 
scribed, although  the  structural  details  are  somewhat  different. 
The  line  wire  enters  at  spring,  U,  and  thence  passes  to  spring,  F, 
through  the  heat  coil,  B,  to  the  spring,  G,  and  thence  to  the 
switch-board  wire  through  terminal,  E.  The  spring,  F,  rests  in  a 
groove  of  the  carbon  block,  A,  which  is  separated  from  a  similar 
block  by  a  small  strip  of  mica,  shown  in  detail  at  the  right- 
hand  portion  of  this  figure.  This  second  block  rests  on  a 
ground  plate.  An  added  feature  of  protection  is  provided  by 


Fig.  229. — Rolfe  Arrester. 

inserting  in  one  of  these  carbon  blocks  a  small  drop  of  fusible 
metal.  If  an  arc  occurs  between  the  two  blocks,  this  metal  will 
melt,  thus  establishing  a  perfect  connection  between  the  two,  and 
grounding  the  line.  In  case,  however,  a  smaller  current  comes 
in  over  the  line,  it  operates  the  heat  coil  and  allows  the  central 
rod,  which  is  of  metal  in  this  case,  to  press  the  light  spring  at- 
tached to  the  lower  side  of  F  into  connection  with  the  ground 
plate.  The  other  side  of  the  line  is  connected  to  the  other  side 
of  the  switch-board  coil  in  the  same  manner  ;  the  line  entering 
at  the  terminal,  C,  passes  by  means  of  an  insulated  bolt  through 
the  iron  fram  e  on  which  the  apparatus  is  placed,  to  and  through 
the  corresponding  heat  coil  on  the  lower  side  of  the  plate.  It 
passes  to  the  switch-board  wire  by  the  terminal,  E. 


280  AMERICAN   TELEPHONE  PRACTICE. 

Still  another  type  of  arrester  which  is  coming  into  increasing 
use  among  the  independent  companies  is  that  shown  in  Fig.  229. 
In  this,  which  is  the  invention  of  Mr.  C.  A.  Rolfe  of  Chicago, 
the  two  binding  posts  which  are  connected  respectively  to  the 
clips,  D,  form  the  terminals  of  the  line  wire,  and  of  the  wire  lead- 
ing to  the  instrument  to  be  protected.  On  an  insulating  strip,  C, 
usually  of  fiber,  are  provided  the  metal  ends,  c,  which  are  adapted 
to  be  held  firmly  between  the  clips,  D.  A  fine-wire  coil,  E,  of 
German  silver  is  connected  between  the  metal  end  pieces,  c,  its 
terminals  being  attached  thereto  by  small  screws  and  washers. 
The  coil,  E,  is  imbedded  in  a  mass,  G,  of  some  easily  fusible 
substance  resembling  plumbers'  wax ;  the  smaller  portion  of 
which  extends  through  an  eye,  //,  on  the  plate,  C.  This  eye  is 
arranged  to  support  G,  and  to  provide  a  stop  against  which  the 
head  of  the  button  is  normally  held  by  the  tension  of  a  spring,  /, 
secured  to  the,  upper  portion  of  the  plate,  C,  as  shown.  A  is  a 
coiled  spring  mounted  upon  the  base,  K,  and  provided  with  an 
arm,  a,  which  may  be  held  by  a  catch,  B.  The  relative  positions 
of  the  springs,  /and  A,  are  such,  that  if  the  spring,  /,  is  released, 
it  will  strike  the  arm,  a,  of  the  spring,  A,  and  cause  it  to  disen- 
gage the  catch,  B.  The  spring,  A,  will  then,  in  its  attempt  to 
rise,  as  indicated  in  the  dotted  portion  of  Fig.  229,  strike  the 
under  side  of  the  plate,  C,  and  lift  it  entirely  out  of  the  clips,  D. 
As  the  coil,  E,  forms  a  part  of  the  circuit,  a  current  in 
excess  of  that  which  it  is  adapted  to  carry  will  develop  enough 
heat  to  melt  the  wax,  and  this  will  allow  the  head  of  the  coil 
to  pull  through  the  ring,  H.  This  in  itself  usually  breaks  the 
wire,  E,  and  thereby  opens  the  circuit ;  but,  as  an  additional 
protection,  the  spring,  A,  gives  a  violent  kick,  which  is  sufficient 
to  throw  the  entire  plate,  C,  and  its  mechanism  high  into  the 
air.  This  affords  a  very  effective  break  between  the  line  ter- 
minals— so  much  so  that  is  almost  impossible  for  an  arc  to  form. 
This  arrester  is  sometimes  termed  the  grasshopper  cut-out,  on 
account  of  its  peculiar  action. 


CHAPTER  XXIV. 

DISTRIBUTING   BOARDS. 

IN  every  central  office  some  means  must  be  provided  for  dis- 
tributing the  various  line  wires  which  enter  the  exchange  to  their 
proper  numbers  on  the  switch-board  and  to  enable  changes  to  be 
made  in  this  distribution  as  required.  If  such  provision  were  not 
made,  and  the  line  cables  were  run  directly  to  the  switch-board, 
the  wires  in  one  one-hundred-pair  cable,  for  instance,  being  led 
to  the  No.  I  section,  and  those  of  another  to  the  No.  2  section 
of  the  switch-board,  and  so  on,  it  would  be  necessary,  at  any  time 
when  a  change  in  a  subscriber's  number  was  desired,  to  open  the 
cable,  take  out  the  proper  wire  and  fasten  it  alongside  of  one  of 
the  other  cables  leading  to  the  proper  section  of  the  board. 
The  changing  about  of  wires  from  one  part  of  a  board  to  another 
is  a  very  frequent  occurrence,  and  to  do  it  in  the  manner  above 
suggested  would  be  entirely  impracticable.  To  do  it  in  any 
manner  without  a  proper  regard  to  systematic  arrangement 
would  lead  to  endless  trouble,  by  producing  a  tangle  of  wires, 
commonly  and  well  termed  a  u  rat's  nest." 

In  order  to  provide  means  for  the  systematic  arrangement  of 
the  wires,  what  is  called  a  distributing  board  or  frame  is  used. 
These  assume  a  great  variety  of  forms,  but  the  principle  on  which 
they  are  designed  is  as  follows :  on  one  side  of  the  distributing 
board  are  placed  clips,  suitably  arranged,  in  which  wires  of  the 
line  cables  may  terminate.  On  the  other  side  of  the  distributing 
board  is  arranged  another  set  of  clips  or  connectors,  in  which  the 
separate  wires  of  the  switch-board  cables  may  terminate.  Sup- 
pose, for  convenience,  that  the  cables  entering  an  exchange 
are  twenty  in  number,  each  consisting  of  one  hundred  pairs  of 
wires.  These  wires  would  pass  through  suitable  office  cables  to 
the  various  terminals  on  the  line  side  of  the  distributing  board. 
Suppose  further  that  the  switch-board  was  arranged  in  twenty 
sections  of  one  hundred  drops  each.  Then  twenty  switch-board 
cables,  of  one  hundred  pairs  each,  would  lead  from  the  switch- 
board terminals  to  the  terminals  on  the  switch-board  side  of  the 
distributing  board.  This  brings  connections  from  the  line  cables 
and  also  from  the  switch-board  cables,  in  a  permanent  manner,  to 


282 


AMERICAN   TELEPHONE   PRACTICE. 


the  various  connectors  on  the  respective  sides  of  the  distributing 
board. 

The  gap  between  the  terminals  of  any  pair  on  the  line  side  of 
the  distributing  board  and  that  of  the  corresponding  pair  of  wires 
leading  from  the  switch-board  is  filled  by  means  of  "  bridle  "  or 
"jumper  "  wires.  Suppose  that  the  line  circuit  terminating  in 
terminal  No.  101  on  the  line  side  of  the  distributing  board  is  to 
be  connected  with  drop  and  jack  No.  599  on  the  switch-board  ; 


Fig.  230. — End  View  Hibbard  Distributing  Board. 

then  a  bridle  wire  is  run  from  terminal  No.  101  on  the  line  side 
to  terminal  No.  599  on  the  switch-board  side,  thus  completing  the 
circuit  of  that  line  between  the  switch-board  drop  and  the  sub- 
scriber. 

One  side  of  the  distributing  board  often  carries  lightning  ar- 
resters through  which  the  various  line  circuits  pass  before  enter- 
ing the  switch-board.  These,  however,  are  sometimes  placed  on 
a  separate  board  between  the  line  side  of  the  distributing  board 
and  the  cable  heads.  Test  clips  are  also  often  provided  on  one 
side  of  the  distributing  board.  These  are  usually  simple  forms 
of  jacks,  normally  maintaining  the  continuity  of  the  lines.  They 
are,  however,  adapted  to  receive  a  test  plug  so  that  the  testing 
operator  may  connect  his  testing  apparatus  with  the  line  side  of 
the  circuit,  leaving  the  switch-board  side  open,  or  with  the  switch- 


DISTRIBUTING  BOARDS. 


283 


board  side  of  the  circuit,  leaving  the  line  side  open,  or  he  may 
merely  bridge  his  testing  apparatus  between  the  two  sides  of  the 
line  without  breaking  its  continuity. 

Inasmuch  as  there  are  in  a  large  exchange  several  thousand  of 
these  bridle  wires,  means  are  provided  for  their  systematic 
arrangement  as  far  as  possible.  The  chief  object  of  distributing 
boards  is  to  bring  all  of  the  confusion  among  the  wires  leading 
from  the  subscribers  to  the  switch-board  into  one  small  place, 
and  then  to  minimize  that  confusion  as  much  as  possible.  This 


i  U  U  \  I  k  V  U  U  i  I  \\\ 

I  ^^^^=J^^^=^^^^^ 

Fig.  231. — Side  Elevation  Hibbarcl  Distributing  Board. 

is  well  done  in  the  Hibbard  distributing  board,  modifications  of 
which  are  used  to  a  large  extent  in  the  Bell  exchanges. 

This  was  designed  by  Mr.  Angus  S.  Hibbard,  and  is  illustrated 
somewhat  in  detail  in  the  accompanying  figures..  The  frame  is 
built  up  entirely  of  iron  pipes,  extending  in  three  directions  and 
mounted  upon  a  hollow  platform,  a,  shown  in  Figs.  230  and  231. 
These  two  figures  represent  respectively  the  end  and  side  eleva- 
tions of  the  complete  framework,  a  plan  view  being  shown  in  Fig. 
232.  Vertical  pipes  serve  as  supports  for  the  structure,  and  are 
intersected  at  short  intervals  by  transverse  pipes,  z,  and  longi- 
tudinal pipes,  e,  /,  and  g,  extending  the  entire  length  of  the 
framework.  As  a  result  of  this  arrangement  channels  or  horizontal 
runs  are  formed  for  the  jumper  wires  between  the  vertical  and 
the  lateral  bars,  and  vertical  channels  or  falls  between  the  sets  of 
intersecting  horizontal  bars.  On  the  ends  of  the  lateral  bars,  i, 


284 


AMERICAN   TELEPHONE  PRACTICE. 


are  vertical  strips,  d  and  d',  of  insulating  material,  upon  which  are 
arranged  the  terminals  for  the  various  wires  in  the  cables  and 
the  jumpers. 

The  general  plan  by  which  the  wires  are  led  from  the  cable 
heads  to  the  switch-board  is  shown  quite  clearly  in  Fig.  230,  where 


t 

f            j 

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^ 

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arcl. 

H  represents  the  cable  head  carrying  the  terminals  of  the  line 
cable,  C.  The  various  wires,  w,  leading  from  the  cable 
head  are  bunched  into  a  cable,  £72,  which  enters  the  cable  run  in 
the  box  beneath  the  frame,  and  after  passing  in  a  horizontal 
direction  to  the  proper  insulating  strip,  d',  is  led  upward  and 
fanned  out,  the  various  pairs  of  wires  being  soldered  to  the  outer 
ends  of  the  terminals  on  the  insulating  strip.  The  method  of 
fanning  out  is  shown  in  Fig.  233,  the  covering  of  the  cable  being 


Fig.  233. — Method  of  Fanning  Out  Cables. 

taken  off  and  the  various  pairs  of  wires,  r  r,  being  led  out  at  in- 
tervals corresponding  to  the  distance  apart  of  the  terminals  on 
the  strip.  After  being  properly  formed  the  cable  is  laced  and 
varnished  or  coated  with  beeswax,  after  which  it  is  strapped  into 
place  and  the  wires  soldered  to  the  terminals  on  the  strip. 

The  details  of  these  strips  and  the  method  of  attaching  the 
wires  of  the  cable  are  shown  in  Fig.  234,  in  which  /  and  /'  are 
the  connectors  screwed  to  the  strip,  d.  These  connectors  have 
outwardly  bent  lugs,  u,  to  which  the  wires  may  be  soldered. 
The  ends  of  the  jumper  wires  are  shown  at  1 1'.  In  a  similar 
manner  the  wires  leading  from  the  switch-board  jack  are  bunched 


DISTRIBUTING  BOARDS. 


285 


into  a  cable,  C*y  which  is  then  led  through  the  cable  run  and  to 
the  proper  strip,  d,  of  the  distributing  board,  where  it  is  fanned 
out  and  connected  to  similar  terminals.  The  vertical  portions 
of  the  cables,  which  are  to  be  fanned  out  on  the  distributing  board, 


Fig.  234. —Enlarged  Plan  Hibbard  Board. 

are  supported  by  the  lateral  horizontal  rods,  i,  by  being  laced  there- 
to, this  being  shown  quite  clearly  in  the  enlarged  plan  view  of 
Fig.  235.'  The  jumper  wires,  which  are  usually  formed  of  No.  22 
B.  &  S.  gauge  tinned  rubber-covered  wire  in  twisted  pairs,  are  at- 


Fig.  235. — Detail  of  Connection  Strips. 

tached  to  the  inner  ends  of  the  terminals  on  the  line  side  of  the 
distributing  board  and  led  through  a  hole  in  the  strip  and  through 
the  proper  channels  in  the  framework  to  the  desired  terminals 
on  the  switch-board  side,  where  they  are  secured  in  the  same 
manner. 

This  arrangement  serves  to  keep  the  wires  fairly  open  and  easy 


286 


AMERICAN    TELEPHONE  PRACTICE. 


of  access,  but  for  very  large  exchanges  it  becomes  cumbersome, 
and  has  been  supplanted  by  one  designed  by  Messrs.  Ford  & 
Lenfest,  some  of  the  details  of  which  are  shown  in  Figs.  236,  237, 


SWITCHBOARD. <C  /  \ 
CABLE       Ux         u 


Fig.  236. — Ford  &  Lenfest  Distributing  Board. 


and  238.  This,  like  the  Hibbard  board,  is  in  the  form  of  an 
open  framework  built  chiefly  of  iron.  Iron  bars,  I,  2,  and  3,  to 
which  are  bolted  plates,  4,  form  the  foundation  of  the  frame. 
To  the  face  of  the  plates,  4,  are  bolted  the  supporting  columns,  D ', 
of  angle  iron,  to  which  are  secured  all  of  the  other  portions  of 


DIS TRIE  U  77 'NG  BOA  RDS. 


287 


the  frame.  Horizontal  bars,  6,  are  bolted  to  the  columns,  D\ 
and  carry  upon  one  side  of  the  frame  horizontal  strips,  8,  of  hard 
wood,  upon  which  are  secured  the  terminals  for  the  wires  of  the 


Fig.  237. — Ford  &  Lenfest  Distributing  Board. 

street  cables.  A  detail  of  these  terminals  is  shown  in  Fig.  238, 
the  metallic  connectors,  m  and  n,  being  secured  in  place  in  trans- 
verse saw-cuts  in  a  thin  strip  of  board  by  another  strip  bolted 


Fig.  238.— Line  Terminals,  Ford  &  Lenfest  Distributing  Board. 

over  them.  A  good  idea  of  this  general  construction  may  also 
be  had  from  Fig.  239,  which  shows  a  slightly  modified  construc- 
tion. Supported  upon  the  other  end  of  the  horizontal  bars,  6, 


288 


AMERICAN   TELEPHONE  PRACTICE. 


are  the  vertical  pieces,  10  and  n,  of  hard  wood  and  the  flat  bar,. 
12,  which  is  of  iron.  Upon  this  bar  of  iron  are  mounted  the  ar- 
resters, X  and  Y,  as  shown  in  Fig.  237.  These  arresters,  which 
are  of  the  combined  static  and  sneak-current  type,  will  be  recog- 
nized as  the  same  as  one  of  those  shown  in  Fig.  228. 

In  wiring  this  distributing  frame,  the  street  cables,  C,  are  led 
in  a  horizontal  direction  under  the  strips,  6,  as  shown  in  Figs.  236 


»*«** 


Fig.  239. — Line  Terminals,  Ford  &  Lenfest  Distributing  Board. 

and  238.  These  cables  are  then  fanned  out,  the  various  pairs  of 
wires  passing  through  holes,  14,  in  the  under  side'of  the  horizon- 
tal wooden  strip,  8,  and  secured  to  the  lower  ends  of  the  con- 
nectors, m  and  n.  The  switch-board  cables,  Z,  shown  in  Figs.  236 
and  237,  are  led  from  beneath  up  along  the  sides  of  the  bars,  6, 
between  the  supporting  bars,  D',  and  the  wooden  strips,  10  and 
ii.  They  are  supported  in  this  position  by  being  laced  to  the 
horizontal  bars  themselves.  These  cables  are  fanned  out,  the 
various  pairs  passing  through  holes,  15  and  16,  in  the  wooden 
strips,  10  and  11,  and  to  their  appropriate  terminals  on  the 
arresters.  The  connections  of  the  street  and  switch-board  cables 
are  thus  as  far  as  possible  made  permanent.  The  jumper  wires 
are  each  led  through  a  hole,  13,  in  the  upper  part  of  the  horizon- 
tal wooden  strip,  8,  its  ends  being  secured  to  the  upper  portion 
of  the  connectors,  m  and  ?z,  as  shown  in  Fig.  238.  The  pair  is 
then  led  in  a  horizontal  direction  along  the  top  of  the  bars,  6,  on 


DISTRIB  U TING  BOA RDS. 


289 


the  line  side  of  the  frame  until  a  point  is  reached  opposite  the 
vertical  strip  on  which  the  desired  switch-board  terminal  is  located. 
It  is  then  led  through  an  eye  or  ring,  e,  and  through  holes,  17, 
in  the  vertical  strip,  u,  and  attached  to  the  proper  pair  of  ter- 


Fig.  240. — Horizontal  Side  St.  Louis  Distributing  Board. 

minals  on  the  arrester  through  which  the  connection  is  made 
with  the  switch-board  wires. 

A  distributing  frame  built  upon  this  general  plan  is  shown  in 
Figs.  240  and  241. 

The  line  cables  enter  the  exchange  and  are  fanned  out  on  the 


290 


AMERICAN   TELEPHONE  PRACTICE. 


horizontal  side  of  the  distributing  frame,  as  shown  in  Fig.  240. 
The  terminals  on  the  line  side  are  numbered  with  respect  to  .the 


Fig.  241. — Vertical  Side  St.  Louis  Distributing  Board. 

wires  in  the  cables  to  which  they  belong.  On  the  vertical  side 
of  this  board,  which  is  shown  in  Fig.  241,  are  placed  the  arresters, 
to  which  lead  the  wires  from  the  switch-board  cables.  The 


292  AMERICAN   TELEPHONE   PRACTICE. 

jumper  wires  connecting  the  horizontal  with  the  vertical  sides 
are  arranged  as  already  described. 

In  this  exchange,  and  in  many  other  of  the  more  modern  Bell 
exchanges,  the  wires  from  the  vertical  side  of  the  distributing 
frame  do  not  pass  directly  to  the  switch-board,  but  to  an  inter- 
mediate distributing  frame  which  is  similar  to  the  one  shown, 
with  the  exception  that  the  arresters  are  left  off.  After  passing 
through  this  intermediate  distributing  frame  the  cables  are  again 
formed  up  and  led  to  the  switch-board.  At  the  main  distribut- 
ing board  all  of  the  changes  are  effected  which  are  made  neces- 
sary by  the  change  in  the  location  of  the  subscribers,  by  the 
addition  of  new  subscribers  and  the  loss  of  old  ones.  The  func- 
tion of  the  intermediate  distributing  board  is  to  permit  of  the 
rearrangement  of  the  lines  upon  the  switch-board,  in  order  that 
they  may  be  grouped  to  the  best  advantage  for  quick  service. 
By  it  the  number  of  calls  received  per  hour  by  the  various  oper- 
ators may  be  practically  equalized  so  that  a  part  of  the  operators 
will  not  be  overworked,  due  to  having  an  undue  proportion  of 
busy  lines. 

The  distributing  board  manufactured  by  the  Western  Tele- 
phone Construction  Company  for  some  of  its  larger  exchanges  is 
shown  in  Fig.  242.  The  side  shown  in  this  figure  is  the  line  side, 
to  which  the  line  wires  from  the  cable  heads  are  run.  Each  of 
the  vertical  strips  seen  in  the  lower  section  of  the  structure  con- 
tains lightning  arresters  for  twenty-five  metallic  circuits.  These 
arresters  consist  merely  of  delicate  fuses  with  suitable  clips  for 
holding  them.  Each  strip  is  provided  with  a  ground  plate  com- 
ing into  close  proximity  with  the  various  clips,  so  that  a  high- 
tension  charge  may  find  passage  to  the  ground.  The  cables  lead- 
ing from  the  cable  heads  of  the  outside  lines  are  afforded  room 
in  the  box  or  trough  underneath  this  structure,  and  each  one  is 
bent  upward  when  opposite  the  proper  pair  of  strips  and  then 
fanned  out  and  permanently  connected  to  the  proper  clips 
on  the  lightning  arrester  strips.  All  of  these  connections  are 
made  permanent. 

In  like  manner  the  cables  leading  from  the  switch-board  are 
brought  into  the  same  trough  and  bent  upward  through  holes  in 
the  baseboard  on  the  opposite  side  from  the  line  cables,  and  are 
then  fanned  out  and  connected  to  the  test  clips,  which  are  ar- 
ranged on  vertical  strips  similar  to  those  containing  the  lightning 
arresters.  These  connections  are  also  made  permanent.  Each 
jumper  wire  is  run  from  the  proper  clips  on  the  arrester  side 
under  the  nearest  one  of  the  horizontal  wooden  rods  shown  on 


DISTRIBUTING  BOARDS.  293 

the  interior  of  the  lower  part  of  the  structure,  and  then  bent  up- 
ward so  as  to  pass  through  a  hole  to  a  rack  above.  Inasmuch  as 
the  yire  is  to  go  to  a  certain  terminal  on  the  switch-board  side,  it 
passes  through  a  corresponding  opening  in  the  rack  and  thence 
in  a  horizontal  channel,  formed  by  the  outwardly  projecting  pins, 
until  it  is  opposite  the  terminal  t;o  which  it  belongs  on  the  switch- 
board side  of  the  board.  Here  the  pair  of  jumper  wires  again 
passes  through  the  vertical  rack  and  down  through  a  small  open- 
ing and  under  one  of  the  horizontal  rods  below,  after  which  it  is 
soldered  to  the  proper  pair  of  terminals.  This  completes  the  con- 
nection from  the  outside  line  to  the  switch-board  line. 

The  test-clips  on  the  opposite  side  of  the  boards  and  lightning 
arrester  clips  are  of  heavy  German-silver  springs  arranged  for 
the  insertion  of  a  double  plug  in  such  manner  that  the  testing 
apparatus  may  be  connected  in  any  desired  manner  to  any  line. 

The  best  wire  to  use  for  jumpers  is  No.  20  or  22  B.  &  S.  gauge 
tinned  rubber-covered,  twisted  in  pairs.  It  is  convenient  to  have 
the  two  wires  forming  a  pair  of  different  colors,  so  as  to  distin- 
guish between  the  tip  and  sleeve  sides  of  the  line. 


CHAPTER  XXV. 

PARTY   LINES — NON-SELECTIVE. 

PROBABLY  no  branch  of  telephone  work  has  offered  more  advan- 
tages to  the  inventor  and  designer,  and  consequently  received 
a  greater  share  of  ingenious  application,  than  the  party-line 
problem. 

A  party  line  is  a  line  having  more  than  two  stations  upon  it. 
This  definition  probably  needs  a  little  explanation,  as  a  line  run- 
ning from  a  central  office  to  two  stations  only  is  a  party  line,  and 
we  must  therefore  count  the  central  office  as  a  station,  thus  mak- 
ing three  in  all.  The  term  party  line  is  used  in  distinction  from 
private  line,  which  may  be  defined  as  a  line  connecting  a  central 
office  with  one  subscriber  only,  or  one  subscriber  with  one  other 
only. 

Party  lines  may  be  divided  into  two  general  classes : 

(1)  Those  where  a  code  of  audible  signals  is  employed  to  enable 
the  various  parties  to  distinguish  their  calls  from  those  of  others. 

(2)  Those  where  a  system  of  selective  signaling  is  employed  so 
that  any  one  party  may  be  called  up  without  disturbing  any  of  the 
others. 

The  first  of  these  classes  may  be  divided  into  two  general  sub- 
classes, according  to  the  connection  of  the  instruments  on  the  line, 
as  follows  : 

(a)  Those  on  which  the  instruments  are  connected  in  series  in 
the  line  circuit. 

(b)  Those  on  which  the  instruments  are  connected  in  multiple  in 
the  line  circuit. 

The  second  or  selective  signaling  class  of  lines  may  be  divided 
into  three  sub-classes,  according  to  the  method  of  selective  signal- 
ing used,  as  follows : 

(a)  Those  employing  step-by-step  movements  to  complete  the 
desired  circuit. 

(b)  Those   using   currents   of    different   strengths   or   different 
polarities,  or  both,  for  operating  the  different  signals. 

(c)  Those   using   the    harmonic   system   of   selecting — that   is, 
those  using  currents  of  various  frequencies  for  actuating  the  dif- 
ferent signals. 


PAR  T Y  LINES— NON-SELECT!  VE. 


295 


The  non-selective  systems  will  be  first  considered. 

Probably  the  first  party  line  ever  constructed  connected  the 
instruments  in  the  line  circuit  in  series ;  there  are  records,  how- 
ever, in  the  very  early  days  of  telephony,  of  their  connection  in 
multiple. 

In  the  series  party  line  the  usuaj  form  of  wiring,  such  as  is  shown 
in  Fig.  87,  is  used.  Instruments  of  this  kind  are  connected 
directly  in  the  line  circuit,  that  is;  the  line  wire  is  cut  and  the  two 
terminals  so  formed  are  connected  to  the  two  binding  posts,  I  and 
2.  In  other  words,  the  line  circuit  enters  one  binding  post  of  the 
instrument,  passes  through  the  circuits  of  the  instrument,  and  out 
at  the  other  binding  post  and  to  the  next  instrument,  and  so  on 
through  the  entire  circuit. 

A  grounded  line  of  four  such  instruments  is  shown  in  Fig.  243. 


Fig.  243. — Series  Grounded  Line. 

This  figure  simply  illustrates  the  method  of  connecting  the  tele- 
phones in  the  line  wire,  it  being  understood  that  all  of  the  instru- 
ments are  wired  substantially  in  accordance  with  Fig.  87. 

A  little  consideration  will  now  show  one  of  the  chief  disadvan- 
tages of  the  series  line.  The  talking  circuit  of  'any  two  stations 
engaged  in  conversation  must  always  pass  through  the  bell  mag- 
nets of  all  the  other  stations.  As  these  magnets  necessarily  pos- 
sess considerable  impedance,  this  is  a  very  serious  objection,  and 
when  a  great  number  of  instruments  are  used  the  talking  becomes 
very  faint.  For  this  reason  it  is  customary  to  wind  the  bell  mag- 
nets on  instruments  to  be  used  on  series  lines  to  a  low  resistance, 
rather  lower  in  fact  than  on  the  ordinary  exchange  instruments. 
Eighty  ohms  for  each  complete  double  magnet  is  a  very  good 
resistance,  the  winding  being  of  No.  31  B.  &  S.  gauge  single  silk- 
insulated  copper  wire. 

It  might  be  thought  at  first  sight  that  the  resistance  of  the 
armatures  of  the  magneto-generators  would  also  be  included  in 


296 


AMERICAN    TELEPHONE   PRACTICE. 


the  circuit.  This  was  true  in  the  earliest  forms  of  instruments, 
and  proved  a  most  serious  objection.  Now  every  good  series 
instrument  is  provided  with  an  automatic  shunt,  which,  as  has 
been  shown,  provides  that  a  path  of  practically  no  resistance  shall 
always  be  closed  about  the  generator  armature,  except  at  such 
times  as  the  generator  is  being  operated. 

The  number  of  bells  that  can  be  rung  on  a  series  line  is  far  in 
excess  of  the  number  that  can  be  talked  through.  Thus  fifty 
instruments  would  have  a  combined  resistance  of  4000  ohms,  and 
if  we  assume  a  very  high  line  resistance  of  3000  ohms,  we  have  a 
total  resistance  of  only  7000  ohms,  which  a  good  generator  could 
ring  through.  Fifty  instruments  in  series,  however,  or  even  half 


Fig.  244. — Bridged  Grounded  Line. 

that  number,  without  line  resistance,  give  almost  intolerable  talk- 
ing service. 

Such  a  line  as  that  shown  in  Fig.  243  would,  moreover,  be  sus- 
ceptible to  all  the  inductive  trouble  to  which  the  telephone  is 
heir.  This  can,  of  course,  be  partly  remedied  by  making  the  cir- 
cuit a  complete  metallic  one,  and  transposing  the  line  at  frequent 
intervals ;  but  even  this  will  not  do  away  with  the  trouble  alto- 
gether, as  it  is  impossible  to  get  a  proper  balance  between  the  two 
sides  of  the  circuit. 

The  generators  for  series  instruments  should  be  wound  for  pro- 
ducing a  high  electromotive  force,  inasmuch  as  there  is  always  a 
great  amount  of  resistance  to  be  overcome.  A  good  type  of 
generator  is  one  wound  with  No.  35  single  silk-covered  wire  to  a 
resistance  of  550  ohms.  Such  a  generator,  with  proper  mechani- 
cal construction  and  good  permanent  magnets,  will  easily  ring 
through  15,000  ohms. 

It  is  well  to  explain  here  what  is  meant  by  the  terms  "  ten 
thousand  ohm  "  or  "  twenty-five  thousand  ohm  "  generators.  It 


PARTY  LINES— NON-SELECTIVE. 


297 


means  that  the  generator  will  ring  its  own  bell  through  the 
resistance  specified. 

The  bridging  or  multiple  system  of  party-line  working  is  now 
rapidly  superseding  the  series  system.  Fig.  244  shows  the  method 
of  attaching  the  telephones  to  a  single  or  grounded  line  accord- 
ing to  this  plan. 

The  line  wire,  /,  is  continuous  through  all  the  stations,  and  each 
instrument  is  placed  in  a  separate  bridge  wire,  b,  or  tap  to  ground. 


Fig.  245.— Eleven-Station  Bridged  Party  Line. 

If  the  circuit  is  to  be  metallic,  the  ends  of  the  bridge  wires,  b, 
which  are  shown  connected  with  the  ground,  are  connected  instead 
with  the  second  line  wire. 

The  circuits  of  a  bridging  instrument  are  shown  in  Fig.  89,  and 
the  line  connections  of  an  eleven-station  metallic-circuit  line  in 
Fig.  245.  This  latter  figure  is  a  reproduction  of  a  figure  in  the 
famous  Carty  patent  on  bridging  telephones.  The  various  instru- 
ments, 2,  3,  4,  5,  etc.,  are  connected  across  the  two  sides,  /and  /*, 
of  the  line  wire,  L.  If  the  station  is  not  located  directly  on  the 


298  AMERICAN    TELEPHONE   PRACTICE. 

route  of  the  line,  it  is  connected  in  by  running  lateral  wires,  13 
and  14,  from  its  binding  posts  to  the  line  wires. 

In  this  system  the  call-bells,  Pt  at  each  station  are  permanently 
bridged  across  the  two  sides  of  the  line,  and  are  made  of  high 
resistance  and  retardation.  The  generator,  G,  at  each  station  is 
in  a  separate  bridge  circuit,  which  is  normally  open,  but  closed 
when  the  generator  is  operated.  The  talking  circuit  of  each 
instrument,  containing  the  receiver,  R,  and  secondary  winding  of 
the  induction  coil,  /,  forms  a  third  bridge  circuit,  which,  like  the 
generator  circuit,  is  normally  open. 

The  telephone  circuit  of  each  instrument  is  automatically  closed 
when  the  receiver  is  removed  from  its  hook  for  use,  and  this 
operation  also  closes  the  local  circuit  containing  the  primary  of 
the  induction  coil,  /,  the  local  battery,  b,  and  the  transmitter,  T. 
In  order  that  there  shall  not  be  an  undue  leakage  of  the  voice 
currents  through  the  permanently  bridged  call-bell  circuits,  the 
magnets  of  these  call-bells  are  wound  to  a  high  resistance  (usually 
a  thousand  ohms)  and  are  also  constructed  in  such  manner  that 
they  will  have  a  high  coefficient  of  self-induction.  When  a 
generator  at  any  one  station  is  operated,  it  is  connected  across 
the  two  sides  of  the  line  in  parallel  with  all  of  the  call-bell  mag- 
nets on  the  line.  Part  of  the  currents  in  this  generator  will, 
therefore,  pass  through  each  of  the  call-bell  magnets  on  the  line, 
thus  causing  them  all  to  operate  if  the  amount  of  the  current 
generated  is  sufficient  to  accomplish  this  result.  The  successful 
operation  of  this  system  depends  on  the  fact  that  a  coil  possess- 
ing a  high  coefficient  of  self-induction  will  transmit  with  com- 
parative ease  alternating  or  pulsating  currents  of  low  frequency, 
while  it  will  form  a  practical  barrier  to  similar  currents  having  a 
very  high  frequency.  The  currents  generated  by  the  calling 
generator  at  any  station  are  of  sufficiently  low  frequency 
to  pass  with  comparative  ease  through  the  call-bell  magnets 
arranged  along  the  line,  while  the  rapidly  alternating  voice 
currents  impressed  upon  the  line  by  the  telephonic  apparatus  at 
any  station  will  be  compelled  to  pass  over  the  main  line  to  the 
receiving  station  without  being  materially  weakened  by  leakage 
through  the  call-bell  magnets.  At  the  receiving  station  these 
voice  currents  will  pass  through  the  telephone  receiver  and 
secondary  coil  of  the  induction  coil,  these  being  connected  across 
the  line  at  that  station  by  virtue  of  the  receiver  being  off  its  hook. 
Thi£  path  through  the  receiving  instrument  is  of  comparatively 
low  resistance  and  retardation,  and  thus  practically  takes  all  of 
the  current  from  the  distant  station. 


PARTY  LINES— NON-SELECTIVE.  299 

The  closing  of  the  generator  bridge  upon  the  sending  of  a  call 
may  be  accomplished  manually,  as  with  the  key,  ky  in  Figs.  89 
and  245,  or  automatically,  in  much  the  same  manner  as  that 
described  for  breaking  the  shunt  around  the  generator  in  the 
series  instrument. 

The  high  retardation  of  the;  ringer  magnets  is  obtained  by 
winding  them  to  a  high  resistance  with  a  comparatively  coarse 
wire  so  as  to  obtain  a  large  number  of  turns  in  the  winding. 
The  length  of  the  cores  is  increased  for  the  double  purpose  of 
getting  more  iron  in  the  magnetic  circuit,  and  therefore  a  higher 
retardation,  and  also  for  affording  a  greater  amount  of  room  for 
the  winding.  The  Western  Electric  Company  wind  their  coils 
to  a  resistance  of  1000  ohms,  using  No.  33  single  silk  magnet 
wire.  Many  other  companies  use  No.  38  wire  and  wind  to 
a  resistance  of  1200  or  1600  ohms.  This  does  not  give  such 
good  results,  however,  as  using  the  coarser  wire  and  the  lower 
resistance  and  long  cores.  Some  companies  wind,  or  once 
wound,  their  bridging  bell  magnets  partly  with  German-silver 
wire  in  order  to  make  a  high  resistance  at  a  low  cost.  They 
should  learn,  however,  that  resistance  in  itself  is  not  the  thing 
desired,  but  a  great  number  of  turns  in  the  winding,  which,  of 
course,  incidentally  produces  a  high  resistance. 

The  generators  for  bridging  systems  should  be  designed  for 
quantity  of  current  rather  than  high  pressure,  since  they  have  to 
supply  current  to  pieces  of  apparatus  arranged  in  multiple.  The 
fact  that  in  some  instances  a  high  voltage  also  is  needed  must 
not  be  overlooked.  On  long  iron  lines,  heavily  loaded,  sufficient 
current  must  be  generated  to  ring  all  the  bells  in  multiple  and 
sufficient  voltage  to  ring  the  bells  at  the  farthest  end  of  the  line. 
In  this  case  it  becomes  a  question  of  watts,  horse-power,  or,  more 
properly,  man-power.  The  winding  of  the  generator  is,  there- 
fore, a  question  of  vital  importance  and  must  vary  to  meet 
different  requirements.  A  generator  wound  to  350  ohms  with 
No.  33  wire  makes  a  first-class  one,  however,  for  ordinary  bridged 
lines  where  copper  circuits  are  employed. 

It  is  undoubtedly  better  on  bridged  circuits  to  use  low-wound 
induction  coils,  so  that  the  voice  currents  coming  along  the  line 
wire  will  find  a  much  readier  path  through  the  talking  circuit  of 
the  station  receiving  than  through  the  call-bell  bridges  at  the 
various  stations.  In  many  cases  the  use  of  500-  and  even  1000- 
ohm  induction  coils  dn  bridged  circuits  renders  the  impedance 
of  the  talking  circuits  very  high,  which  is  exactly  what  should  be 
avoided. 


300  AMERICAN    TELEPHONE  PRACTICE. 

In  connecting  a  party  line  with  a  switch-board  much  trouble 
is  often  caused  by  the  use  of  an  improperly  wound  annunciator 
coil.  It  should  be  borne  in  mind  that  the  drop  magnet  really 
bears  the  same  relation  to  the  line  as  the  ringer  magnets,  in  the 
various  telephones,  and  should  therefore  be  connected  in  the 
same  way.  For  a  series  party  line  the  switch-board  drop  should 
be  wound  to  about  the  same  resistance  as  the  ringer  magnets. 
If  the  resistance  is  made  higher,  as  is  often  done  in  the  attempt 
to  secure  a  more  sensitive  drop,  the  parties  on  the  line  will  have 
much  difficulty  in  talking  to  each  other,  because  the  drop  is  in 
series  in  the  line;  but  if  that  line  is  connected  with  some  other 
line,  through  the  switch-board,  this  trouble  will  not  exist,  as  the 
circuits  should  be  so  arranged  as  to  cut  out  the  drop  upon  the 
insertion  of  the  plug. 

In  the  bridging-bell  system  the  resistance  of  the  switch-board 
drop  should  also  be  about  the  same  as  that  of  the  ringer  mag- 
nets, and  it  should  possess  a  high  coefficient  of  self-induction,  so 
as  to  prevent  the  short-circuiting  of  the  voice  currents.  It  is  fre- 
quently impossible,  however,  to  wind  drops  to  1000  ohms  on  ac- 
count of  insufficient  wire  space,  and  in  this  case  the  tubular  drop 
wound  to  500  ohms  should  be  used.  A  properly  designed  bridged 
drop  may  be  left  permanently  bridged  across  the  line,  to  serve 
as  a  clearing-out  drop  when  the  subscribers  are  through  talking. 
In  small  exchanges,  operating  party  lines,  it  is  customary  for  the 
operator  at  such  a  switch-board  to  distinguish  between  the  calls 
for  a  connection  with  some  other  line,  and  those  which  are  for 
parties  on  the  same  line,  by  means  of  the  buzz  caused  by  the 
vibration  of  the  armature  of  the  drop.  It  is,  therefore,  desirable 
to  give  the  drop  armature  a  rather  wide  adjustment,  so  that  it 
will  make  enough  noise  to  enable  the  operator  to  readily  dis- 
tinguish the  signals. 

On  lines  where  a  measured  service  rate  is  charged,  much  loss 
of  revenue  is  often  caused  by  surreptitious  conversations,  that  is, 
by  parties  on  the  same  line  calling  each  other  and  carrying  on 
their  conversation  without  the  knowledge  of  the  switch-board 
operator,  so  that  no  means  is  afforded  for  properly  charging  the 
use  of  the  line  against  them.  Many  arrangements  of  circuits 
and  apparatus  have  been  devised  for  obviating  this  difficulty. 
One  of  these,  which  is  suitable  only  for  bridging  lines,  is  to  pro- 
vide at  the  central  office  a  switch-board  drop  of  extremely  low 
resistance  and  so  arrange  it  that  it  will  be  cut  out  upon  the 
insertion  of  the  plug.  The  low-resistance  path  through  this 
drop  acts  practically  as  a  short-circuit  to  all  of  the  high  resist- 


PARTY  LINES— NON-SELECTIVE.  301 

ance  bells  on  the  line,  so  that  when  any  party  rings,  nearly  all 
of  the  current  from  his  generator  passes  through  the  switch- 
board drop,  without  actuating  any  of  the  bells.  When  the 
operator  plugs  in  for  conversation,  or  for  the  purpose  of  calling 
up  some  subscriber  on  that  line,  the  low-resistance  drop  is  cut 
out,  so  that  the  line  is  no  longer  short-circuited.  This  method 
cannot  be  used  on  long  lines,  because  the  resistance  of  the  drop, 
in  addition  to  that  of  the  line  wire,  proves  high  enough  to  shunt 
some  of  the  current  through  the  magnets  of  the  bells  at  the 
distant  end  of  the  line,  when  parties  at  that  end  attempt  to 
signal  each  other.  While  the  drop  would  short-circuit  the  end 
of  the  line  nearest  the  switch-board,  the  instruments  at  the 
farther  end  would  not  be  appreciably  affected,  owing  to  the  high 
resistance  of  the  line  wire  between  them  and  the  board. 

This  method  is  not,  on  the  whole,  very  satisfactory,  and  a 
better  one  is  to  arrange  the  magnetos  at  the  subscribers'  stations 
to  generate  a  current  in  one  direction  only,  instead  of  the  usual 
alternating  current,  and  to  give  the  armatures  of  the  bridged  call- 
bells  at  all  of  the  stations  a  permanent  set  or  tendency  toward 
the  pole  which  would  be  rendered  stronger  by  currents  in  this 
direction.  The  switch-board  drop,  also  bridged  across  the  line, 
is  of  a  non-polarized  type,  so  as  to  fall  when  actuated  by  currents 
in  either  direction.  Thus,  when  any  subscriber  calls,  the  current 
will  have  no  effect  upon  any  of  the  ringer  magnets  of  the  other 
subscribers,  because  it  tends  only  to  pull  the  armatures  closer  to 
the  poles  toward  which  they  are  already  attracted,  but  will  cause 
the  switch-board  drop  to  fall  in  the  ordinary  manner.  Thus,  no 
subscriber  can  obtain  a  conversation  with  any  other  subscriber 
without  the  full  knowledge  of  the  operator.  The  switch-board 
generator  is  equipped  for  sending  out  currents,  either  of  the 
opposite  polarity  from  those  generated  by  the  subscribers'  gener- 
ators or  of  the  ordinary  alternating  character,  so  that  the  opera- 
tor may  ring  up  the  subscribers  at  will. 

LOCK-OUT   SYSTEMS. 

A  very  interesting  class  of  systems,  designed  to  secure  a  certain 
degree  of  secrecy  in  party-line  service,  has  come  into  existence 
during  the  last  few  years.  In  the  systems  so  far  described 
there  is  nothing  to  prevent  one  subscriber  from  taking  his  re- 
ceiver off  the  hook  and  listening  to  whatever  conversation  other 
subscribers  may  be  engaged  in.  The  lock-out  systems,  as  they 
are  called,  are  designed  to  remedy  this  evil  and  also  have  for  their 


302 


AMERICAN   TELEPHONE  PRACTICE. 


object  the  prevention  of  subscribers  desiring  to  use  their  instru- 
ments from  breaking  in  while  the  line  is  already  busy,  thus  ring- 
ing in  the  ears  of  the  parties  who  are  using  their  telephones. 

Mr.  C.  E.  Scribner  of  the  Western  Electric  Company  has,  as 
in  nearly  every  other  branch  of  telephony,  been  well  to  the  front 
in  this  line.  One  of  these  systems  designed  by  Mr.  Scribner  is 
shown  in  Fig.  246,  which  illustrates  three  subscribers'  stations,  E, 


Fig.  246. — Scribner  Lock-out  Party  Line. 

E\  and  E* ,  connected  by  the  line  wires,  5  and  6,  of  a  metallic  cir- 
cuit with  the  switch-board  at  the  central  office,  F. 

The  mechanism  for  operating  the  lockout  devices  at  each 
station  on  the  party  line  is  shown  in  Fig.  247.  In  this  figure  a 
magnet,  a,  supported  on  a  bracket,  a1,  is  provided  with  an  arma- 
ture, a1,  carried  upon  a  lever,  a3,  pivoted  as  shown.  The  arma- 
ture, a*,  is  normally  pulled  away  from  the  core  of  the  magnet,  a, 
by  the  attraction  of  gravity,  the  magnet  being  mounted  with  its 
core  vertical.  The  backward  movement  of  the  lever  is  limited 
by  the  stop,  a*,  and  the  forward  movement  by  the  contact  anvil, 
a\  with  which  it  makes  contact  when  the  armature  is  attracted. 

Mounted  alongside  of  the  magnet,  a,  is  a  similar  magnet,  b, 
having  its  armature,  b\  mounted  on  the  short  arm,  b1,  of  a 
bell-crank  lever,  b\  The  armature  is  normally  held  away  from 
the  core  of  the  magnet,  b,  by  the  spring,  £5,  which  bears  against 
the  adjustment  screw,  b".  When  the  armature,  b\  is  attracted 


PARTY  LINES— NON-SELECTIVE. 


3°3 


by  its  magnet,  the  long  arm,  b\  which  normally  rests  against  the 
back  stop,  b\  is  pushed  sidewise  and  into  the  path  of  the  lever, 
#3,  so  as  to  prevent  the  upward  movement  of  the  latter. 

The  hook-switch  is  of  the  Warner  type,  and  the  contacts  are 
so  adjusted  that  the  spring,  c\  makes  contact  with  the  lever  at 
the  point,  c\  before  the  spring,  c\  makes  contact  at  the  point,  c\ 
when  the  receiver  is  removed  from  the  outer  end  of  the  hook. 
The  action  of  these  springs  is  the  same  as  in  the  ordinary  re- 
ceiver hook,  being  such  that  when  the  hook  is  depressed  the 


Fig.  247. — Lock-out  Mechanism. 

spring,  c\  breaks  contact  with  c\  resting  on  the  hard-rubber  lug, 
cj,  while  the  spring,  c1,  breaks  contact  with  c\  and  rests  on  the 
hard-rubber  lug,  c*. 

Referring  now  to  Fig.  246,  and  remembering  that  the  various 
parts  in  the  apparatus  shown  at  the  substations,  E,  E\  and  E*, 
bear  the  same  reference  letters  as  those  in  Fig.  247,  the  circuits 
may  be  traced  as  follows :  The  telephone  switch-hook,  C,  is  per- 
manently connected  to  ground  by  the  wire,  I.  The  wire,  2, 
leading  from  line  wire,  6,  includes  the  winding  of  the  magnet,  b, 
and  terminates  in  the  contact  point,  c2,  which,  it  must  be  remem- 
bered, is  the  contact  first  made  when  the  hook  is  raised.  The 
wire,  3,  which  branches  from  the  main  line  wire,  5,  includes  the 
winding  of  the  magnet,  a,  and  terminates  in  the  contact-spring, 
c\  The  wire,  4,  branches  from  the  wire,  2,  and  includes  the  re- 
ceiver, dlt  and  the  transmitter,  d,  and  terminates  in  the  contact 
point,  a6,  with  which  the  locking  lever,  #8,  makes  contact  when 
attracted  by  the  magnet,  a.  The  apparatus  at  all  of  the  sub- 


3°4  AMERICAN    TELEPHONE  PRACTICE. 

scribers'  stations  on  the  line  are  connected  in  the  same  manner. 
The  main-line  wires,  5  and  6,  terminate  respectively  in  the 
springs,  g  and  g\  of  the  spring-jack.  The  spring,  g,  normally  rests 
on  the  anvil,  ^2,  which  forms  the  terminal  of  a  wire  leading 
through  the  self-restoring  drop,  i,  and  the  battery,  k,  to  ground. 

The  operator's  circuit  is  shown  at  F,  /and  /'  being  respectively 
the  answering  and  calling  plugs  of  a  pair.  The  tips  of  the  plugs 
are  connected  through  the  wire,  7,  while  the  sleeves  are  simi- 
larly connected  through  the  wire,  8  ;  this  latter  wire  includ- 
ing serially  the  clearing-out  or  supervisory  signals,  o  and  o' . 
The  conductors,  7  and  8,  include  each  two  helices,  m  m'  and 
m*  ms,  respectively.  The  point  between  the  coils,  m  and  m',  is 
connected  to  one  terminal  of  a  battery,  n,  while  the  opposite  ter- 
minal of  the  battery  is  connected  to  the  junction  of  the  coils, 
m9  and  m9.  The  arrangement  is  such  that  the  coils,  m  and  m"1, 
act  inductively  on  the  coils,  m'  and  m*,  and  vice  versa.  When  a 
plug  is  inserted  into  a  jack,  therefore,  the  battery,  n,  is  bridged 
across  the  line,  and  thus  supplies  current  directly  for  operating 
the  telephone  transmitters  and  receivers  at  the  substations. 

The  apparatus  is  shown  in  its  normal  or  idle  condition  ;  that 
is,  with  the  plugs  withdrawn  from  the  jacks  and  with  all  of  the 
subscribers'  receivers  resting  upon  their  respective  switch-hooks. 
Suppose,  now,  that  a  subscriber  at  station,  E,  desires  to  be  con- 
nected with  some  other  subscriber ;  he  removes  his  receiver 
from  its  hook,  and  the  latter  in  rising  makes  contact  first  with 
the  point,  <:Q,  and  immediately  thereafter  with  the  point,  c' '. 
The  making  of  the  Contact  with  the  point,  r2,  produces  no  result 
on  the  magnet,  b,  because  there  is  no  battery  in  circuit  with  the 
line  wire,  6,  with  which  the  wire,  2,  is  connected.  As  soon,  how- 
ever, as  the  contact  with  c1  is  made,  a  current  flows  from  the 
battery,  k,  through  the  coil  of  the  drop,  z,  thereby  actuating  the 
shutter  ;  thence  through  the  contact,  g*,  and  spring,  g,  of  wire,  5  ; 
thence  through  the  magnet,  a,  and  wire,  3,  to  contact,  c\  and  to 
ground,  which  forms  the  return  circuit  of  the  battery.  This 
current,  besides  actuating  the  shutter  at  the  central  office,  causes 
the  lever,  a3,  to  come  in  contact  with  the  point,  a6,  thus  complet- 
ing the  circuit  between  the  two  sides  of  the  line  through  the 
telephone  apparatus  proper.  The  lever,  d\  is  allowed  to  rise,  for 
the  reason  that  the  magnet,  b,  has  not  actuated  its  armature  to 
pull  the  lever,  b\  into  the  path  of  the  lever,  a*. 

The  operator  at  the  central  station  seeing  the  shutter  fall,  in- 
serts the  plug,  /,  into  the  spring-jack,  thus  establishing  connection 
with  the  line,  and,  at  the  same  time,  breaking  the  connection  be- 


PARTY  LINES—  NON-SELECTIVE.  305 

tween  the  line  wire,  5,  and  the  drop,  i.  The  operator's  talking 
apparatus  is  not  shown,  but  it  is  adapted  to  be  bridged  across  the 
cord  circuit,  7  and  8,  in  a  manner  well  understood.  It  will  be 
noticed  that  no  induction  coil  is  used  at  the  subscribers'  stations, 
the  current  from  battery,  n,  passing  directly  through  the  trans- 
mitter and  receiver  in  series.  'JThis  circuit  may  be  traced  as 
follows  :  starting  at  the  upper  pole  of  the  battery,  n,  the  current 
passes  through  coil,  n?,  wire,  8,  annunciator,  o,  sleeve  of  plug,  /, 
sleeve-spring,  g,  of  the  jack,  line  wire,  5,  lever,  a*y  at  the  subscrib- 
er's station,  E,  contact  point,  a\  wire,  4,  transmitter,  d,  receiver, 
d  ',  line  wire,  6,  tip  spring,  g  ,  at  the  central  office,  tip  of  the  plug, 
/,  wire,  7,  and  coil,  m,  to  the  other  pole  of  the  battery,  n. 

The  subscriber  then  communicates  with  the  central  office  in 
the  ordinary  manner,  and  is  there  connected  with  some  other 
subscriber  in  the  exchange  by  means  of  the  plug,  /'.  Suppose, 
now,  that  while  the  subscriber  at  E  is  using  his  telephone,  the 
subscriber  at  £'  desires  also  to  use  the  line  ;  he  removes  his  re- 
ceiver from  its  hook,  and  as  before  the  lever  of  the  hook  first 
makes  contact  with  £2,  and  later  with  c\  As  soon  as  the  contact 
is  made  with  £2,  however,  the  magnet,  b,  at  that  station  attracts 
its  armature  and  pushes  the  stop-controlling  lever,  b\  into  the 
path  of  the  circuit-controlling  armature,  a*.  The  circuit  through 
this  magnet,  #,  may  not  be  at  first  apparent,  but  may  be  traced 
as  follows  :  from  the  line  wire,  6,  through  the  magnet,  b,  at 
station,  E',  to  contact,  £2,  and  to  ground  ;  thence  to  the  ground 
at  station,  E,  where  the  receiver  is  also  off  its  hook,  and  through 
the  contact  point,  c\  at  that  station  and  magnet,  a,  to  the  wire  5. 
Current  is  supplied  to  this  circuit  from  battery,  //.  Since  the 
lever,  a\  at  station,  E\  cannot  rise,  it  is  impossible  to  complete 
the  circuit  through  the  telephone  apparatus  at  that  station  at 
the  point,  a6,  and  it  is  thus  impossible  for  the  subscriber  at  that 
station  or  at  any  other  station  to  use  his  telephone  until  the  sub- 
scriber at  E  has  finished  his  conversation. 

If  the  subscriber,  E\  had  attempted  to  use  the  line  after  the 
subscriber  E  had  removed  his  receiver  from  the  hook,  but  before 
the  operator  at  the  central  office  had  inserted  the  plug  into  the 
jack,  the  same  state  of  conditions  would  have  obtained,  except 
that  the  source  of  current  would  have  been  from  the  battery,  k  ; 
for  when  the  subscriber  at  E  removed  his  receiver  from  its 
hook,  the  battery,  k,  became  connected  with  the  wire,  6,  thus 
making  the  conditions  such  that  when  the  receiver  at  any  other 
station  was  removed  from  its  hook,  the  magnet,  b,  at  that  station 
would  operate  its  lever  to  lock  the  apparatus. 


UNIVERSITY 


3o6 


AMERICAN   TELEPHONE  PRACTICE. 


The  call-sending  apparatus  at  central  office  and  the  call- 
receiving  apparatus  at  the  subscribers'  stations  are  not  shown, 
but  such  calling  is  accomplished  by  the  use  of  the  ordinary 
bridging  bells. 

When  the  subscriber  at  station,  E,  has  finished  his  conversa- 
tion he  replaces  the  receiver  on  its  hook  in  the  ordinary  manner. 
This  breaks  the  connection  which  exists  between  the  two  sides 
(5  and  6)  of  the  line,  and  therefore  stops  the  flow  of  the  current 
from  the  battery,  n.  This  allows  the  shutter  of  the  clearing-out 
drop,  o,  to  fall,  it  having  been  raised  automatically  by  this  cur- 
rent when  the  connection  was  established.  This  shows  the 
operator  that  a  disconnection  is  desired.  As  soon  as  the  sub- 
scriber, who  is  connected  by  the  plug,  I1,  hangs  up  his  receiver, 
the  shutter,  ol,  falls  in  a  similar  manner,  thus  indicating  to  the 
operator  that  both  lines  are  free. 

This  system  is  instructive  in  many  ways.  It  not  only 
embodies  a  very  ingenious  method  for  securing  privacy  on  party 
lines,  but  also  exhibits  the  features  of  automatic  calling  on  the 
part  of  the  subscriber,  and  of  the  centralized  transmitter  batteries 
already  described. 

Fig.  248  illustrates  diagrammatically  a  mechanism  for  use  on 
circuits  practically  the  same  as  those  in  the  system  just  described, 


Fig.  248. — Busy  Signal  for  Lock-out  System. 

with  the  added  feature  that  a  signal  is  automatically  displayed  for 
indicating  to  a  subscriber  when  the  line  is  in  use  at  some 
other  station.  In  this  the  stop-controlling  lever,  represented 
in  this  figure  by  f\  carries  also  a  catch  or  hook,  d\  which 
normally  engages  a  lever,  b,  which  carries  a  target  marked 
"  Busy."  Assuming  that  the  line  is  not  busy,  any  subscriber 
who  raises  his  receiver  from  the  hook  will  obtain  control 
of  the  line,  as  described  in  the  previous  system.  The  magnet, 


PARTY  LINES— NON-SELECTIVE.  307 

f,  however,  not  having  current,  will  not  release  the  lever, 
b,  and  will  thus  hold  the  target  in  its  concealed  position,  even 
though  the  hard-rubber  lug,  a1,  on  the  hook-lever  allows  it  to 
rise.  If  while  the  line  is  busy,  however,  a  second  subscriber 
attempts  to  use  it,  the  raising  of  his  receiver  will  withdraw  the 
lug,  a1,  from  engagement  with  the  lever,  by  and,  in  the  manner 
already  described,  the  magnet,  /,  will  take  current.  This  will 
not  only  lock  the  lever,  g\  but  will  also  withdraw  the  catch,  dl, 
from  engagement  with  the  lever,  b,  and  allow  the  busy  signal  to 
rise.  Thus  the  subscriber  will  not  only  be  locked  out,  but  will 
be  notified  of  that  fact  by  the  signal.  Upon  the  replacement  of 
the  receiver  on  the  hook,  the  lug,  a1,  serves  to  restore  the  busy 
signal,  thus  doing  away  with  all  magnetic  resetting  devices. 


CHAPTER  XXVI. 

PARTY   LINES. — "  STEP   BY   STEP  "    SELECTIVE   SIGNALING. 

WE  come  now  to  the  consideration  of  selective  signaling  on 
party  lines,  and  it  will  be  remembered  that  systems  for  ac- 
complishing this  result  were  divided  into  three  distinct  classes. 
The  first  of  these  classes  includes  those  systems  depending  on 
step-by-step  mechanisms  at  the  subscribers'  stations,  controlled 
from  the  central  station  in  such  a  manner  as  to  enable  the 
operator  to  pick  out  or  select  the  desired  station  and  ring  its 
bell  to  the  exclusion  of  all  others  on  the  same  line.  It  is  well  to 
state  beforehand  that  this  branch  of  party-line  work  will  be  of 
interest  mainly  from  a  historical  standpoint,  and  will  therefore  be 
treated  in  that  light.  There  are  but  few  lines  in  successful 
practical  operation  using  a  system  of  this  class ;  but  this  should 
not  detract  from  the  interest  of  the  subject,  for  there  is  no  doubt 
but  that  the  apparatus  can  be  successfully  operated  on  this  plan, 
especially  in  view  of  the  success  of  the  "  ticker "  and  other 
systems  of  telegraphy  depending  for  their  operation  entirely  on 
step-by-step  movements.  The  use  of  step-by-step  mechanisms 
in  this  class  of  telephone  work  has  apparently  from  the  very  first 
offered  the  most  plausible  solution  of  the  problem,  and  there  are 
seemingly  no  insurmountable  obstacles  in  the  way  of  its  being  put 
into  successful  practice. 

One  of  the  very  first  to  apply  step-by-step  mechanism  to  the 
partly  line  problem  was  E.  N.  Dickerson,  Jr.,  as  early  as 
January,  1879.  His  substation  mechanism  is  shown  in  Fig.  249, 
and  the  line  and  local  circuits  respectively  in  Figs.  250  and  251. 

Referring  now  to  Fig.  249,  B  and  C  represent  two  electro- 
magnets placed  in  series  in  the  line  circuit.  The  armature  of  B 
is  mounted  on  an  arbor,  //,  pivoted  in  the  framework,  A,  as 
shown.  This  arbor  carried  a  lever,  F,  which  is  moved  by  the 
armature,  and  by  means  of  a  pawl,  G,  steps  the  ratchet-wheel,  W, 
around  in  an  obvious  manner.  A  second  pawl,  P,  normally  acts 
to  prevent  a  backward  movement  of  the  shaft,  S,  on  which  the 
wheel,  W,  is  mounted  ;  a  tendency  to  such  backward  movement 
being  given  to  the  wheel  and  shaft  by  the  coiled  spring,  v,  wound 
on  the  shaft. 


PA R  T Y  LINES—  STEP-B  Y-STEP. 


3°9 


The  magnet,  C,  by  the  attraction  of  its  armature,  operates 
upon  the  arm,  M,  pivoted  with  the  armature  upon  the  arbor,  r. 
The  raising  of  this  arm  lifts  both  pawls  out  of  engagement  with 
the  wheel,  W,  allowing  it  to  be  rotated  by  the  spring  until  the 
pin,  b',  engages  the  stop  pin,  g,  when  it  is  in  its  normal  position. 

Upon  the  end  of  the  shaft,  5,  are  two  contact  wheels,  c  and  d, 
upon  which  rest  four  springs,  m  m  and  n  n.  The  periphery 


Fig.  249. — Dickerson  Step-by-Step  Mechanism. 

of  the  wheel,  c,  is  all  of  conducting  material  with  the  exception 
of  two  insulating  strips,  o  and/,  clearly  shown  in  Fig.  250.  The 
wheel,  d,  is  of  the  reverse  construction,  all  of  its  surface  being  of 
insulating  material  with  the  exception  of  the  metallic  contact 
strip,  qj  as  shown  in  Fig.  251.  In  the  normal  position  of  the 
wheel,  c,  at  each  of  the  stations  the  springs,  m  m,  rest  upon  the 
insulating  strip,  o.  The  insulating  strip,  /,  on  the  wheel,  c,  and 
the  conducting  strip,  q,  on  the  wheel,  d,  are  arranged  at  different 
positions  on  the  wheels  of  each  station,  and  always  so  that  when 
the  particular  number  of  impulses  necessary  to  place  the  appara- 
tus at  that  station  in  operative  relation  to  the  line  has  been 
sent,  the  two  strips,  /  and  q,  will  be  respectively  under  the 
springs,  ;/z  m  and  n  n. 

The  apparatus  at  the  central  station  consists  of  batteries  of 
three  strengths :  the  weakest  capable  of  operating  only  a  high- 
resistance  magnet  at  the  central  station;  the  next  stronger 
capable  of  operating  the  magnets,  B,  but  not  the  magnets,  C '; 


3io 


AMERICAN   TELEPHONE  PRACTICE. 


and  the  third,  or  strongest,  of  sufficient  strength  to  operate  the 
magnets,  C.  In  order  to  make  C  responsive  to  the  strongest 
current  only,  the  coiled  spring,  D,  which  controls  its  armature, 
is  given  a  higher  tension  than  the  spring,  £,  controlling  the 


Fig.  250. — Line  Circuit  through  Apparatus. 

armature  of  B.  The  signal-transmitting  apparatus  at  the  central 
stations  consists  of  a  toothed  wheel  or  any  other  device  for  send- 
ing a  predetermined  number  of  impulses  to  the  line,  from  either 


Fig.  251. — Local  Circuit. 

of  the  two  stronger  batteries.     Normally,  the   weakest  of  the 
threte  batteries  is  left  in  line. 

The  normal  condition  of  the  line  circuit  through  a  station  is 
shown  in  Fig.  250,  where  the  springs,  m  m,  rest  on  the  insulating 


PARTY  LINES— STEP -BY-STEP.  311 

portion  of  the  wheel,  c,  and  are  therefore  disconnected  from  each 
other.  The  receiver,  t,  is  shunted  out  of  circuit  by  the  automatic 
hook-switch,  s,  upon  which  it  hangs.  In  this  condition,  there- 
fore, the  circuit  through  the  station  is  from  the  line  through 
magnets,  B  and  C,  in  series,  thence  through  switch,  k,  hook- 
switch,  s,  and  to  line.  The  circuit  is  therefore  complete  when 
no  one  is  using  the  line  from  l ground  at  central,  through  the 
small  battery  and  high-resistance  annunciator  or  bell  at  that 
station,  then  through  all  the  stations  in  series  and  to  ground  at 
the  end  station. 

To  signal  central,  a  party  at  any  station  depresses  the  key,  kf 
momentarily,  thus  breaking  the  circuit  and  releasing  the  arma- 
ture of  the  signaling  magnet  at  central.  The  party  may  then 
communicate  with  central  by  removing  his  telephone  from  its 
hook. 

In  order  for  the  operator  at  central  to  call  up  any  station,  a 
number  of  impulses  from  the  battery  of  intermediate  strength 
is  sent  to  line.  The  first  one  of  these  impulses  advances  all  of 
the  ratchet-wheels  one  step,  and  the  springs,  ;//  ;;/,  therefore  rest 
on  the  conducting  portions  of  the  contact  wheels  at  all  except 
the  first  station,  which  has  the  insulating  strip,/,  so  arranged 
as  to  come  under  the  springs  at  the  first  step.  As  a  result  the 
receivers  at  every  station  except  station  No.  I  are  short-cir- 
cuited through  the  by-path  containing  the  springs,  m  in,  and 
the  disk,  c.  Suppose  the  station  shown  in  Fig.  250  to  be  No.  5, 
then  five  impulses  will  bring  the  strip,  /,  under  the  springs,  m  my 
when  the  receiver  will  be  no  longer  short-circuited. 

At  the  same  time  that  the  strip,/,  comes  under  springs,  m  m, 
the  conducting  strip,  q,  on  the  other  wheel  comes  under 
springs,  n  n,  thereby  completing  the  local  circuit  containing  a 
battery,  £,  and  a  vibrating  bell,  D,  as  clearly  shown  in  Fig.  251. 

The  local  circuit  is  only  closed  when  the  armature  of  magnet,  B, 
rests  against  its  back  stop,  Z.  This  is  to  prevent  the  actuation 
of  the  bells,  D,  at  the  stations  having  a  smaller  number  than  the 
station  desired,  as  it  is  obvious  that  the  springs,  n  n,  will  wipe 
over  the  contacts,  q,  of  all  the  stations  in  succession  which  have 
a  smaller  number  than  the  one  being  called. 

While  this  station  is  engaged  in  conversation  the  other  stations 
are  locked  out  by  reason  of  their  receivers  being  short-circuited. 
At  the  end  of  the  conversation  the  strongest  battery  is  thrown 
on  the  line,  and  the  magnet,  C,  at  each  station  causes  the  arm,  M, 
to  lift  the  pawls,  P  and  G,  in  consequence  of  which  all  the 
ratchets  return  to  their  normal  position, 


312  AMERICAN    TELEPHONE   PRACTICE. 

While  any  good  telephone  man  could  point  out  many  features 
in  this  system  of  an  extremely  objectionable  nature,  such,  for 
instance,  as  the  inclusion  of  so  many  magnets  in  series  in  the 
line,  and  the  employment  of  three  strengths  of  battery  and  a  cor- 
responding marginal  adjustment  of  the  magnets,  the  fact  that 
such  an  ingenious  arrangement  could  be  devised  at  such  an  early 
date  in  the  art  would  seem  to  bode  exceedingly  well  for  future 
development  in  this  line. 

At  almost  the  same  date  George  L.  Anders  produced  a  step- 
by-step  system,  depending  on  a  somewhat  different  idea.  All 
bells  were  left  permanently  in  the  line  wire,  and  their  hammers 
all  actuated  in  unison  when  a  pulsating  current  was  sent  over  the 
line.  A  notched  disk  at  each  station  prevented  the  bell  hammer 
at  its  station  from  striking  the  bell  except  at  such  times  as  the 
notch  was  opposite  the  rod  which  carried  the  hammer.  The  disks 
were  so  arranged  as  to  be  stepped  around  by  the  vibrations  of 
the  bell  hammers  while  impulses  of  one  polarity  were  sent  over 
the  line.  In  calling  a  certain  party  a  sufficient  number  of  im- 
pulses were  sent  to  bring  the  notch  of  the  bell  at  the  desired 
station  into  a  position  opposite  the  bell-hammer  rod,  after  which 
currents  of  the  opposite  polarity  were  sent  over  the  line.  These 
latter  did  not  actuate  the  stepping  device,  but  did  actuate  all 
the  bell  hammers  as  before,  and  the  notch  in  the  disk  of  the  de- 
sired station  allowed  that  bell  to  sound. 

Dickerson  used  his  stepping  device  to  control  a  local  circuit  at 
each  station.  Anders  left  all  his  circuits  unaltered,  and  used  his 
stepping  device  to  control  merely  the  length  of  stroke  of  the  bell 
hammer. 

Still  another  interesting  example  of  the  early  art  in  this  line  is 
the  system  of  Thomas  D.  Lockwood,  designed  early  in  1881, 
which  is  well  illustrated  in  Fig.  252.  In  this  the  toothed  wheels, 
z,  shown  at  the  different  substations  are  all  adapted  to  be  revolved 
by  clockwork  at  exactly  the  same  rate,  so  that  when  they  are  all 
released  at  once  they  will  move  with  the  same  angular  velocity 
until  stopped.  Each  wheel  is  furnished  with  square  teeth,  cor- 
responding in  number  to  the  stations  on  the  circuit.  These  are 
placed  at  a  suitable  distance  apart  on  the  periphery,  and  in  each 
case  one  tooth,/,  of  the  series  is  composed  of  non-conducting  ma- 
terial, which  is  inserted  into  the  metal  portion  of  the  wheel. 
This  non-conducting  tooth,  /,  is,  of  course,  differently  placed  at 
each  station  in  the  circuit,  as  shown  in  the  drawings,  where  the 
central  office  wheel  has  its  insulating  tooth  placed  as  (he  first 
tooth  of  the  series.  In  station  No.  2  it  is  the  second  tooth,  and 


PARTY  LINES— STEP-B Y-STEP. 


313 


so  on.  The  material  of  which  this  tooth  is  formed  also  extends 
forward  for  a  short  distance  toward  the  base  of  the  tooth  in  ad- 
vance, so  that  when  the  lug,  e,  of  the  lever,  /,  strikes  the  insulating 


Fig.  252.—  -Lockwood  Step-by-Step  System. 

tooth  in  any  instrument  it  will  not  touch  the  metallic  part  of 
the  wheel  at  any  point. 

Each  circuit  or  escapement  wheel  is  also  provided  with  an 
extra  tooth,  u,  set  at  a  distance  from  any  of  the  others,  and  when 
the  circuit  is  not  being  used  the  lug,  e,  of  each  lever  will  be  ele- 
vated and  rest  against  the  tooth,  u.  This  tooth,  u,  affords  a  con- 
venient point  at  which  each  wheel  may  come  to  rest,  so  that 
after  each  revolution  all  the  wheels  shall  be  in  unison. 

The  release  magnet,  A,  one  of  which  is  included  in  series  in  the 
line  at  each  station,  forms  a  unique  feature  of  this  system.  It  is 
shown  in  the  small  detail  figure  at  the  left  of  Fig.  252.  A  is  a 


314  AMERICAN    TELEPHONE   PRACTICE. 

permanent  magnet  of  hardened  steel,  to  the  poles  of  which  are  at- 
tached two  soft-iron  pole-pieces,  P  P,  on  each  of  which  is  wound  a 
coil,  a.  The  strength  of  the  permanent  magnet  may  be  adjusted  by 
clamping  the  iron  bar,  q,  at  a  point  nearer  to  or  farther  from  its 
pole-pieces.  The  strength  of  the  magnet  at  each  station  is  so 
adjusted  that  it  will  just  hold  down  the  armature,  B,  mounted  on 
the  retaining  lever  controlling  the  toothed  wheel,  z,  at  that  station. 

The  central  office  is  provided  with  a  battery,  L  B,  and  keys, 
k  k'y  adapted  to  send  a  current  of  either  polarity  to  the  line, 
and  also  with  an  apparatus  similar  to  that  at  each  station,  so  that 
the  operator  may  \vatch  the  positions  of  the  wheels  in  their 
rotation. 

The  operation  may  now  be  readily  understood.  In  order  to 
start  the  wheels  the  operator  depresses  lever,  k ,  and  holds  it  down. 
This  sends  to  line  a  current  of  such  a  direction  as  to  neutralize 
the  polarity  of  each  permanent  magnet,  Ay  so  that  all  the  levers 
are  released,  thus  allowing  all  of  the  wheels  to  start  under  the  influ- 
ence of  their  clockworks.  We  will  say  that  No.  5  is  the  station 
to  be  called.  The  operator  watches  the  revolving  wheel  at  the 
central  station,  and  when  the  number  5  is  under  the  index 
pointer,  n,  she  releases  the  lever,  knowing  that  the  insulating 
tooth  at  station  No.  5  is  then  under  the  lug,  et  on  the  lever  at 
that  station.  The  armatures  of  all  the  magnets  are  thus  re-at- 
tracted and  all  of  the  wheels  again  locked.  The  operator  then 
depresses  key  lever,  kt  which  sends  a  strong  current  of  the  op- 
posite polarity  to  line.  This  does  not  release  the  levers,  as  it 
strengthens  the  magnets,  A,  but  it  does  ring  the  bell,  D,  at  station 
No.  5,  because  the  shunt  which  normally  exists  around  the  bells 
at  each  station  has  been  removed  from  bell  No.  5  by  virtue  of 
the  lever  resting  against  the  insulating  tooth  on  the  wheel.  The 
bells  at  all  the  other  stations  are  short-circuited,  and,  therefore, 
do  not  ring.  The  contacts  c  at  each  station  are  provided  for 
short-circuiting  the  bells  when  the  levers  are  released.  To  bring 
all  the  wheels  again  to  the  normal  position,  with  the  tooth,  u,  of 
each  resting  against  its  lever,  the  operator  depresses  the  releasing 
key  as  before,  and  allows  the  wheels  to  rotate  until  the  tooth,  u, 
is  almost  reached.  Each  wheel  is  then  stopped  at  the  tooth,  u. 

Several  systems  depending  on  the  same  general  principles  as 
this  have  been  devised,  but  none  have  met  with  success,  so  far  as 
I  am  aware.  Much  trouble  is  experienced  in  keeping  the  wheels 
in  synchronism,  and  another,  and  more  serious  difficulty,  is  the 
maintenance  of  the  contacts  in  proper  condition.  This  latter 
feature  occurs  as  a  fault  in  all  step-by  step  systems. 


PARTY  LINES—  S TEP-B  Y-STEP. 


3'5 


The  systems  so  far  described  have  all  related  to  signaling  on 
a  single-grounded  circuit  with  instruments  in  series.  A  simi- 
lar system  devised  by  F.  B.  Wood  in  1888  placed  all  the  step-by- 
step  magnets  in  a  controlling  wire  which  formed  a  complete 
metallic  loop,  and  used  this  circuit  merely  to  govern  the  step-by- 
step  movements  which  completed  the  desired  circuits  succes- 
sively in  a  separate  wire,  over  which  the  signaling  was  accom- 
plished. Space  will  not  permit  of  a  complete  description  of  this 
system,  nor  of  one  invented  by  Mr.  John  I.  Sabin  of  the  Sunset 


Fig.  253. — Reid  &  McDonnell  Bridged  Step-by-Step  System. 

Telephone  Company,  in  San  Francisco.  In  this  latter  system  the 
magnets  of  the  step-by-step  mechanisms  were  placed  in  a  third 
wire  and  used  to  successively  close  bridge  circuits  containing  tele- 
phone instruments  and  call-bells  at  the  various  stations. 

A  more  recent  invention,  by  Messrs.  R.  T.  Reid  and  J.  L. 
McDonnell  of  Tacoma,  Wash.,  is  adapted  for  use  on  two 
wires  only,  and  also  contains  lockout  and  automatic  calling 
features.  This  system  is  illustrated  diagrammatically  in  Fig.  253, 
and  some  of  its  mechanism  shown  in  Fig.  254.  In  Fig.  253  the 
central  office  apparatus,  for  the  purpose  of  clearer  illustration,  is 
shown  in  a  greatly  simplified  form,  the  signal-transmitting  ap- 


AMERICAN    TELEPHONE   PRACTICE. 

paratus  being  represented  by  manual  keys.  The  step-by-step 
mechanisms  are  shown  in  this  figure  at  D,  and  are  bridged  be- 
tween the  line  wire,  A,  and  ground  at  each  station.  The  call-bells, 
C,  are  of  the  usual  polarized  type,  and  are  each  contained  in 
a  normally  open  circuit  between  the  line  wire,  B,  and  ground. 
This  circuit  at  each  station  is  adapted  to  be  closed  by  the  step-by- 
step  movement. 

The  step -by-step  mechanisms  (Fig.   254)  are  actuated  and  con- 
trolled by  two  magnets,  19  and  20,  placed  in  series  and  wound  to 


Fig.  254. — Mechanism  of  the  Reid  &  McDonnell  System. 

respectively  high  and  low  resistances.  The  magnet,  19,  will, 
therefore,  operate  with  a  comparatively  smaller  current  than 
magnet,  20,  owing  to  its  greater  number  of  turns. 

Magnet,  19,  by  means  of  lever,  25,  acts  to  step  the  ratchet- 
wheel,  22,  around,  this  wheel  carrying  with  it  the  contact  arm, 
29,  and  the  stop  arm,  31.  These  parts  are  mounted  on  the 
shaft  as  shown,  the  notched  wheel,  28,  being  provided  merely  to 
secure  a  proper  angular  adjustment  between  the  stop  arms,  31, 
and  contact  arms,  29,  this  adjustment  being  different  at  each 
station.  The  low-resistance  magnet,  20,  operates  a  contact  arm, 
32,  carrying  a  contact,  36,  insulated  therefrom,  and  also  a 
separate  arm,  37,  adapted  to  engage  the  stop  arms,  31,  and  lock 
the  shaft. 

Referring  again  to  Fig.  253,  the  lock-out  mechanism  is  repre- 
sented by  magnets,  74,  and  arms,  76.  The  armature  is  polar- 
ized so  as  to  hold  the  arm,  76,  either  under  the  hook-lever,  F,  or 


PARTY  LINES—  STEP-BY-STEP.  317 

away  from  it,  according  to  the  direction  of  the  current  traversing 
the  coils. 

The  operation  may  now  be  understood.  To  call  central,  the 
subscriber  removes  his  receiver  from  its  hook,  thus  closing  a 
circuit  at  his  station  across  the  two  line  wires  and  throwing  the 
drop,  6,  at  central  by  means  of  the  battery,  7.  This  attracts 
the  attention  of  the  operator.  The  circuit  so  closed  between  the 
two  sides,  A  and  B,  of  the  line  includes  the  magnet,  74,  and  the 
current  is  in  such  a  direction  as  to  throw  the  lever,  77,  to 
the  right,  thus  allowing  the  hook-switch  to  rise  and  complete 
the  telephone  circuit  at  that  station.  After  a  plug  and  cord  are 
attached  to  the  line  at  the  central  station,  a  different  battery  of 
opposite  polarity  is  connected  with  the  line,  and,  should  any 
other  party  remove  his  receiver  from  its  hook,  he  will  find  the 
hook-lever  locked  by  reason  of  this  reversed  battery. 

To  call  any  particular  party,  the  key,  12,  at  central  is  de- 
pressed once  and  then  released.  This  unlocks  all  of  the  lock 
arms,  31,  and  moves  all  wheels  forward  one  step.  After  this  the 
key,  11,  is  depressed  a  certain  number  of  times.  This  throws 
a  series  of  weak  impulses  on  the  line,  which  moves  all  the  con- 
tact arms  in  unison.  When  a  sufficient  number  of  impulses 
have  been  sent,  the  arm,  29,  at  the  desired  station  is  opposite 
the  spring,  36.  The  operator  then  depresses  key,  12,  and  sends 
a  strong  current  to  line  and  thus  operates  magnets,  20.  This 
closes  the  bell  circuit  only  at  the  station  desired,  for  the  reason 
that  the  contact  arms,  29,  at  the  other  stations  are  not  in  the 
proper  position  to  make  contact  with  spring,  36.  The  current 
from  the  calling  generator  is  now  thrown  to  line,  B,  thus  operat- 
ing the  bell  of  subscriber  desired.  After  the  required  signal  has 
been  sent,  key  1  1  is  again  depressed  a  sufficient  number  of  times 
to  bring  all  stop  arms  into  engagement  with  their  levers,  37. 

The  systems  described  in  this  chapter  have  been  chosen  as 
representative  of  a  large  number  of  a  similar  nature.  It  has 
been  thought  best  to  give  a  rather  complete  and  detailed  de- 
scription with  intelligible  diagrams  of  a  few  such  systems,  than 
to  describe  in  a  more  general  way  a  greater  number. 


CHAPTER    XXVII. 

PARTY    LINES. — SELECTIVE    SIGNALING    BY    STRENGTH 
AND    POLARITY. 

THE  term  "  strength  and  polarity "  is  borrowed  from  the 
Patent  Office  nomenclature,  where  it  is  applied  to  that  class  of 
selective  calling  devices  which  depend  for  their  operation  on 
changes  in  the  strength  or  in  the  direction  of  a  current,  or  on 
changes  in  both.  The  idea  of  selective  signaling  by  changes  in 
the  strength  and  polarity  of  a  current  was  well-known  in  teleg- 
raphy before  the  birth  of  the  art  of  telephony.  The  duplex 
and  quadruplex  systems  of  telegraphy  of  the  present  time  afford 
the  best  possible  demonstration  of  the  utility  and  practicability 
of  this  system  when  properly  developed.  In  the  quadruplex, 
one  key  at  each  station  operates  to  produce  changes  only  in  the 
strength  of  the  current,  while  the  other  key  at  each  station  is 
capable  of  producing  changes  only  in  the  direction  of  the  current. 
Also  at  each  station  are  two  relays,  one  termed  the  "  neutral 
relay,"  responsive  to  changes  only  in  the  current  strength,  being 
indifferent  to  changes  in  polarity,  and  the  other  termed  the 
"  polarized  or  polar  relay,"  which  is  responsive  to  changes  in  the 
direction  of  the  current  only,  being  indifferent  as  to  its  strength. 
The  arrangement  is  such  that  the  key  at  one  station  governing 
the  strength  of  current  will  operate  only  the  neutral  relay  at  the 
other  station,  while  the  key  governing  the  direction  of  current 
will  operate  only  the  polarized  relay  at  the  other  station.  This 
system,  therefore,  not  only  admits  of  selective  signals  being  sent 
one  at  a  time,  but  also  allows  four  to  be  transmitted  simultane- 
ously over  a  single  grounded  circuit,  two  in  one  direction  and 
two  in  the  other. 

The  problem  is  somewhat  different  in  telephone  work,  but  the 
same  principles  are  involved,  and  the  success  of  the  quadruplex 
telegraph  demonstates  beyond  question  that  the  strength  and 
polarity  system  can  be  made  thoroughly  practical  in  telephony. 
At  present,  nearly  all  of  the  party  lines  successfully  using  select- 
ive signaling  are  operated  on  this  general  plan. 

Aniong  the  first  to  attack  the  problem  from  this  standpoint 
was  George  L.  Anders,  who  in  1879  produced  a  two-party  line 
system,  having  the  call-bells  at  the  two  stations  polarized  oppo- 


PARTY  LINES— STRENGTH  AND  POLARITY. 


3*9 


sitely,  and  included  serially  in  the  line  wire.  Currents  in  a 
positive  direction  would  therefore  operate  the  bell  at  one  station, 
and  those  in  a  negative  direction  that  at  the  other.  The  call 
bell  was  arranged  with  two  armatures,  one  polarized  and  one 
neutral,  the  latter  serving  to  operate  the  bell  striker,  and  the 
former  serving  simply  as  a  lock  for  the  striking  armature.  The 
bell  would  operate  only  when  the  current  was  of  proper  direction 
to  cause  the  magnet  to  remove'the  locking  armature  from  the 
path  of  the  striking  armature.  The  operator  at  the  central 
station  used  a  double  lever  key  to  send  either  positive  or 


255.  —  Anders  System. 


negative  calling  currents  to  line.  This  was  the  forerunner  of 
several  more  successful  plans  recently  devised. 

The  system  shown  in  Fig.  255  is  interesting  as  being  one  of 
the  early  attempts  to  utilize  changes  of  both  strength  and 
polarity.  It  is  typical  of  many  of  the  early  failures  in  this  line 
of  work,  and  is  not  here  described  because  of  any  practical  ideas 
it  contains. 

Eight  stations  are  connected  in  series  in  the  line,  which  is 
adapted  to  be  grounded  at  central  through  the  various  keys  and 
batteries  as  shown.  Each  of  the  controlling  magnets  consists  of 
a  permanent  horseshoe  magnet  carrying  soft-iron  pole-pieces  and 
bobbins.  The  armature  of  each  magnet  is  normally  attracted  by 


320  AMERICAN    TELEPHONE   PRACTICE. 

the  permanent  magnetism,  and  thus  holds  open  a  local  circuit, 
shown  only  at  the  first  station,  containing  a  battery  and  bell  at 
each  station.  The  magnets  at  stations  I  and  2  exert  an  equal 
pull  on  their  armatures,  but  are  of  opposite  polarity,  and  likewise 
the  two  magnets  in  each  other  pair  of  stations  are  of  equal 
strength,  but  of  opposite  polarity.  The  strength  of  the  magnets 
is,  however,  different  for  each  successive  pair  of  stations — those 
at  stations  I  and  2  being  the  weakest,  and  those  at  stations  7  and 
8  the  strongest.  Four  different  strengths  of  current  of  either 
polarity  may  be  sent  to  line  from  central  by  the  closure  of  the 
various  switches.  The  magnet  at  any  given  station  is  supposed 
to  release  its  armature  only  when  a  current  of  the  proper  strength 
and  direction  is  sent  over  the  line  to  exactly  neutralize  its  per- 
manent magnet.  Thus,  if  it  is  desired  to  call  station  I,  switch 
lever  I  at  central  is  closed.  This  sends  a  positive  current  from 
one  set  of  cells  over  the  line  which  is  of  the  proper  strength  and 
direction  to  neutralize  the  pull  of  the  magnet  at  station  I.  This 
magnet  will  therefore  release  its  armature.  The  armature  at 
station  2  will  not  be  released,  because  the  current  is  in  the  wrong 
direction,  and  therefore  strengthens  the  pull  of  the  magnet. 
The  armatures  at  the  other  stations  will  not  be  released,  because 
the  current  is  not  strong  enough.  In  calling,  say  station  8,  a 
strong  negative  current  would  be  employed.  This  would  more 
than  neutralize  the  magnets  of  stations  2,  4,  and  6,  giving  them 
an  opposite  polarity,  and  thus  still  attracting  their  armatures. 
All  such  systems,  depending  for  their  action  upon  close  marginal 
adjustment  of  the  strengths  of  magnets,  have  proven  failures  in 
practice,  because  of  the  varying  of  conditions  assumed  to  be 
constant. 

Coming  now  to  the  more  practical  systems,  one  due  to  Sabin 
&  Hampton,  and  used  to  some  extent  on  the  Pacific  Coast,  will 
be  considered.  This  is  not  properly  a  strength  and  polarity  sys- 
tem, but  is  described  in  this  place  because  it  contains  several 
ideas  upon  which  later  systems  have  been  based.  The  idea  upon 
which  this  is  based  is  that  three  circuits  may  be  obtained  from 
the  two  wires  of  a  metallic  circuit  by  using  the  two  wires  for  one 
circuit,  one  of  the  wires  and  the  ground  return  for  another,  and 
the  other  wire  and  ground  for  the  third.  In  Fig.  256  two  party 
lines  of  three  stations  each  are  shown  connected  through  a  cord 
circuit  and  the  jacks  and  plugs  of  a  switch-board  at  the  central 
office.  The  circuits  of  one  station  are  shown  in  the  small  de- 
tached portion  of  this  figure.  Between  the  two  limbs  of  the 
metallic  circuit  are  included  the  talking  apparatus,  composed  of 


PARTY  LINES— STRENGTH  AND  POLARITY. 


321 


the  transmitter,  t,  and  receiver,  r,  associated  with  the  induction 
coil,  battery,  and  switch-hook  in  the  ordinary  manner.  The  talk- 
ing circuits  at  all  of  the  stations  are  the  same  as  this,  but  are 
represented  merely  by  a  circle  in  each  station.  The  bell,  b,  of 
subscriber  A  is  included  with  a  condenser  in  a  bridge  circuit  be- 
tween the  two  sides  of  the  line,  the  bell,  b,  of  subscriber  B  is  in- 
cluded in  a  branch  between  the  side,  a,  of  the  line  and  ground, 
while  the  bell,  b,  of  subscriber  C  is  similarly  included  between 


Fig.  256. — Sabin  &  Hampton  Three-Station  Line. 

the  other  side,  a,  of  the  line  and  ground.  The  bells  of  stations 
D,  E,  and  Ft  on  line,  d  d ',  are  similarly  arranged. 

The  limbs,  a  a',  of  the  metallic  circuit  extend  to  the  line 
springs,  a*  a3,  of  a  spring-jack  on  the  switch-board,  which  normally 
rest  against  the  contact  anvils,  a*  a6,  between  which  are  included 
the  battery,  a6,  and  indicator  or  annunciator,  a1. 

The  operator's  telephone  set,  e,  is  included  in  a  normally  open 
bridge  between  the  tip  and  sleeve  strands,  e  e1,  of  the  cord,  a 
key,  e*,  being  provided  for  bridging  the  telephone  into  circuit. 
A  clearing-out  indicator,  /  and  battery,/',  are  included  in  a 
bridge  between  the  two  strands,  a  balancing  coil,/2,  being  also 
located  in  said  bridge.  By  means  of  a  key,  g,  a  generator,  g1, 
may  be  bridged  between  the  strands,  e  and  e\  the  generator,^-2, 
by  means  of  a  key,  g\  may  be  connected  between  the  sleeve 
strand,  e\  and  ground,  while  the  generator,  g\  by  means  of  the 
key,  £"5,  may  be  similarly  connected  between  the  tip  strand,  e't 
and  ground. 

Suppose  subscriber  A  desires  to  converse  with  subscriber  D. 
He  removes  his  telephone  from  its  hook,  thus  completing  the 
circuit,  of  battery,  a\  through  indicator,  a1,  and  thus  calling  the 
attention  of  the  operator,  who  inserts  answering  plug,  h,  in  the 


322  AMERICAN    TELEPHONE   PRACTICE. 

spring-jack,  thereby  cutting  out  battery,  a\  and  indicator,  a\ 
The  operator  then  depresses  the  key,  e3,  thus  bridging  her  tele- 
phone into  circuit  and  receives  the  number  of  the  called  sub- 
scriber D.  She  then  inserts  calling  plug,  //',  in  the  spring  jack 
in  which  the  limbs,  d  d' ,  terminate,  and  depresses  key,  g,  thus 
sending  a  calling  current  from  the  generator,^',  over  the  metallic 
circuit  to  actuate  the  bell,  b,  at  station  D.  Subscriber  D  removes 
his  telephone  from  its  hook  and  A  and  D  are  connected  for  con- 
versation. 

Had  A  desired  connection  with  F  instead  of  D,  the  operator 
\vould  have  depressed  key,  £*,  thus  ringing  the  bell,  b,  at  station 
F,  over  a  circuit  formed  by  the  line  wire,  d,  with  ground  return. 

The  condensers,  <:,  at  stations  A  and  D  are  for  the  purpose  of 
preventing  the  steady  current  from  battery,  a'', .  from  leaking 
through  the  bridges  in  which  the  bells,  b,  at  those  stations 
are  placed.  These  condensers  form  a  break  in  these  bridges 
through  which  an  unvarying  current  cannot  pass,  but  they  allow 
the  alternating  currents  from  the  calling  generator  to  act  induct- 
ively through  them  to  operate  the  bells  as  though  they  were  not 
present. 

Where  three  stations  are  thus  operated  on  a  metallic  circuit, 
much  trouble  occurs,  due  to  the  fact  that  the  two  bells  on  a  line 
at  the  stations  which  are  not  being  called  are  always  in  series  in 
a  circuit  which  forms  a  shunt  to  the  bell  at  the  station  which  is 
to  be  called.  Thus  if  a  generator  current  is  sent  over  the  metallic 
circuit  to  call  station  A,. a  part  of  this  current  will  leak  from 
limb,  a',  through  bell,  b,  at  station  B  to  ground,  thence  to  ground 
at  station  C  and  through  the  bell,  b,  at  that  station  to  the  other 
limb,tf,  of  the  line.  This  bridge  circuit  has  about  twice  the  resist- 
ance of  the  bridge  at  A  (disregarding  the  condenser),  and  this  fact 
must  be  depended  upon  to  prevent  the  bells  at  B  and  C  from 
ringing.  The  same  conditions  exist  in  ringing  either  of  the 
other  bells,  and  this  difficulty  has  rendered  the  use  of  three 
stations  on  a  line,  according  to  this  method,  impracticable  save 
in  rare  cases,  as  it  is  very  difficult  to  so  adjust  the  bells  that  they 
will  respond  only  at  the  proper  times.  Two  stations  on  a  line, 
arranged  as  at  B  and  C,  may,  however,  and  often  do,  give  good 
service.  On  long  lines,  however,  there  is  sometimes  enough 
induction  between  the  two  wires  of  the  metallic  circuit  to  cause 
both  bells  to  ring  when  only  one  is  intended  to  respond. 

A  very  successful  four-station  party-line  system  devised  by  Mr. 
Angus  S.  Hibbard  is  shown  in  Fig.  257.  In  this  system,  as  in 
several  others,  the  idea  first  used  by  Anders,  of  placing  two 


PARTY  LINES— STRENGTH  AND  POLARITY. 


323 


oppositely  polarized  bells  on  a  single  line,  has  been  combined 
with  that  of  Sabin  &  Hampton,  just  described,  of  ringing  over 
different  circuits  formed  by  using  the  separate  limits  of  the  line 
with  a  ground  return. 

At  stations  A  and  B  polarized  bells,  d  and  d ',  are  connected 
between  the  limb,  a,  of  the  lirie  and  ground.  The  bell  at  A 
is  so  polarized  as  to  be  operated  only  by  currents  sent  over  the 
limb,  a,  in  one  direction,  while  the  bell  at  B  will  respond  for  a 


a 


Fig.  257. — Hibbard  Four-Station  Line. 

similar  reason  only  to  currents  in  the  same  limb  in  the  opposite 
direction.  In  like  manner,  the  bells,  e  and  e ,  at  stations  C  and 
D  are  oppositely  polarized  and  connected  between  the  limb,  d, 
and  ground,  so  that  bell,  *?,  will  respond  to  current  sent  over  line, 
a',  in  one  direction,  while  the  other  bell,  e\  will  respond  to  current 
over  the  same  wire  in  the  opposite  direction.  Thus,  any  one  of 
the  four  stations  may  be  called  alone  by  sending  the  current  in 
the  proper  direction  over  one  of  the  two  wires. 

The  line  terminates  at  the  central  station  in  a  spring-jack,/", 
composed  of  line  springs,  /'  and  /2,  normally  resting  against 
anvils,  connected  respectively  to  a  battery,  B ',  and  signal  indicator, 


324  AMERICAN   TELEPHONE  PRACTICE. 

5,  in  substantially  the  same  manner  as  in  the  Sabin  &  Hamp- 
ton system.  In  this  case,  however,  the  signal  indicator  is  an 
incandescent  lamp  adapted  to  be  lighted  by  current  from  the 
battery,  B\  when  the  receiver  at  any  station  on  the  line  is  removed 
from  its  hook. 

Four  ringing  keys,  I,  2,  3,  and  4,  are  associated  with  the  plugs,  g 
and  g,  in  such  manner  as  to  enable  the  operator  to  connect  the 
terminal  of  either  of  two  grounded  generators,  o  and  /,  with 
either  the  tip  or  sleeve  strand  of  the  cord,  and  therefore  with 
either  side,  d  or  a,  of  the  line  into  the  jack  of  which  plug,  gt  is 
inserted.  The  generators,  o  and  /,  have  opposite  poles  grounded, 
and  we  will  say  generator,  o,  is  adapted  to  send  positive  impulses 
to  line,  and  generator,  p,  negative  ones. 

When  it  is  desired  to  ring  the  bell  at  station  A  key  No.  I  is 
depressed,  thus  closing  the  circuit  of  generator,  o,  through  con- 
tact, /i\  spring,  h ',  sleeve-strand,^2,  plug,  g,  spring,/2,  limb,  ay 
bell,  d,  to  ground  and  back  to  the  generator.  A  portion  of  the 
current  also  passes  through  bell,  d',  to  ground,  but  as  this  bell 
is  polarized  to  respond  only  to  negative  currents  it  remains 
irresponsive.  Should  it  be  desired  to  ring  the  bell  at  substation 
B,  key  No.  2  is  depressed,  thus  sending  a  negative  current  from 
generator,  /,  to  line,  a,  through  ft",  k ,  h*,  ti ,  strand,  g*,  and  by  the 
same  path  as  before  through  bells,  d  and  d',  to  ground.  Only 
bell,  d',  will  operate,  because  d  is  responsive  only  to  positive 
currents.  In  like  manner  stations  C  and  D  may  be  called  by 
pressing  keys  No.  3,  or  No.  4. 

When  the  key  No.  I,  for  instance,  is  depressed  to  connect  the 
generator,  o,  in  circuit  with  the  limb,  a,  and  ring  the  bell,  d,  the 
spring,  /i*,  is  brought  into  engagement  with  grounded  contact,  /*6, 
thus  grounding  the  strand,  ^3,  and  the  limb,  a,  and  preventing 
the  accidental  ringing  of  the  bell,  e,  should,  for  instance,  one  of 
the  telephone-receivers  be  removed  from  its  hook  and  a  path 
thus  provided  to  the  limb,  d '.  The  current  thus  finds  a  short 
path  to  ground  over  the  limb,  a,  strand,  ^*3,  and  grounded 
spring,  /i6,  and  sufficient  current  will,  therefore,  not  pass  through 
the  bell,  e,  to  ring  it. 

Instead  of  providing  four  keys  in  each  cord  set,  a  single  set  of 
keys  is  usually  provided,  adapted  to  be  connected  by  a  suitable 
switch  with  any  particular  pair  of  cord  conductors  that  may  be 
for  the  time  in  use. 

This  system  with  slight  modifications  is  used  in  a  number  of 
Bell  exchanges  and  apparently  is  a  success.  The  practice  of 
putting  the  lamp  signal  directly  in  the  line  circuit  has,  however, 


PARTY  LINES— STRENGTH  AND  POLARITY. 


325 


not  proven  very  satisfactory,  even  in  cases  where  a  separate 
metallic  circuit  serves  each  subscriber.  Accidental  crosses  or 
grounds  on  the  line  expose  the  lamps  to  higher  voltages  than 
intended,  thus  .frequently  causing  burn-outs.  On  party  lines 
another  difficulty  arises,  due  to  the  difference  in  the  resistance 
of  the  circuit  when  closed  at  thq  different  stations,  owing  to  the 
resistance  of  the  line  wire  between  the  stations.  Obviously  the 
circuit  formed  by  removing  the  receiver  at  A  is  of  less  resistance 
and  will  therefore  expose  the  lamp  to  a  greater  voltage  than  that 


Fig.  258.— McBerty  Four-Station  Line. 

formed  by  removing  the  receiver  at  D.  This,  however,  could  be 
compensated  for  by  winding  the  receivers  of  the  nearer  stations 
higher  than  those  at  the  farther,  or  by  inserting  compensating 
resistance  coils,  but  such  arrangements  are  undesirable. 

Another  system  using  exactly  the  same  method  of  selective 
signaling,  but  employing  a  very  ingenious  arrangement  of 
apparatus  for  carrying  it  out,  is  one  devised  by  Mr.  F.  R. 
McBerty  of  the  Western  Electric  Company.  This  is  shown  in 
Fig.  258.  The  bells  at  each  of  the  stations  and  also  the  talking 
apparatus  are  arranged  with  respect  to  the  two  wires  of  the 


326  AMERICAN    TELEPHONE   PRACTICE, 

metallic  circuit  in  precisely  the  same  way  as  in  Hibbard's  system. 
As  a  safeguard  to  prevent  the  bells  ringing  by  the  wrong  direc- 
tion of  current,  a  light  spring  acts  on  the  pivoted  armature  of 
each  to  retain  the  armature  normally  in  the  position  toward 
which  it  would  be  attracted  by  a  current  in  a  direction  not 
intended  to  operate  the  bell.  The  operation  of  signaling  central 
is  identical  with  that  already  explained. 

In  connection  with  the  line  conductors,  I  2,  are  four  spring- 
jacks,  A ,  B' ,  C,  and  D '.  Each  of  these  has  a  short  line  spring,  n, 
a  long  spring,  n ',  and  a  tubular  thimble,  ;/2.  The  connection  of 
these  springs  and  thimbles  to  the  conductors  of  the  line  is  dif- 
ferent in  the  case  of  each  jack,  as  examination  will  readily  show. 

The  switch-board  is  provided  with  the  usual  plugs,  o  d ,  form- 
ing the  terminals  of  a  plug-circuit,  5  6,  which  includes  a  calling 
key,/.  This  key,/,  in  addition  to  the  pair  of  switch  springs,/'/2, 
and  their  normal  and  alternate  contact  anvils,  has  a  spring,  /3, 
which  is  adapted  to  register  with  an  anvil,/4,  when  the  spring  is 
thrust  outward.  The  spring,  /3,  constitutes  the  terminal  of  a 
contact  piece,  02,  of  the  calling  plug,  o,  which  is  constructed  to 
register  with  the  ring,  «a,  of  a  spring-jack  into  which  the  plug 
may  be  inserted.  The  anvils,/5/6,  of  springs,  /'  and  /2,  con- 
stitute the  terminals  of  a  generator,  ^,  of  alternating  currents. 
This  generator  is  due  to  Scribner,  and  is  of  peculiar  construction. 
It  has  an  armature  of  the  ordinary  type,  one  of  whose  terminals 
is  grounded  permanently,  and  the  other  of  whose  terminals  is  led  to 
a  semi-cylindrical  commutator,  q ',  which  rotates  between  two  con- 
tact springs,  q*  and  g\  These  springs  are  so  placed  with  relation 
to  the  point  at  which  the  direction  of  the  current  in  the  armature 
is  changed  that  spring,  q*,  receives  in  each  revolution  a  pulsation 
of  positively  directed  current,  and  the  spring,  ^3,  during  the  other 
half  of  the  revolution  a  negatively  directed  pulsation.  The 
operation  of  the  key,/,  therefore  always  connects  the  positive 
spring  of  the  generator  with  the  tip  strand,  6,  the  negative  spring 
with  the  sleeve  strand,  5,  and  at  the  same  time  connects  the  plug 
contact,  02,  with  the  ground.  The  arrangements  of  the  jacks 
with  respect  to  the  line  wires  are  such  that  the  mere  insertion  of 
the  plug,  <?,  in  any  jack  will  establish  the  proper  relations  between 
the  generator  and  the  line,  to  operate  the  bell  at  the  correspond- 
ing station  upon  the  depression  of  key,  /.  Thus  suppose  the 
operator  wishes  to  call  station  A.  She  inserts  the  plug  in  jack,  A', 
of  that  line, , and  depresses  key,/.  A  pulsatory  current  in  a 
positive  direction  will  now  flow  from  the  spring,  q\  through  the 
contact  points,/5/2,  thence  through  conductor,  6,  of  the  plug 


PARTY  LINES— STRENGTH  AND  POLARITY.  327 

circuit  to  line  conductor,  I,  and  thence  through  branch  4  and 
bell,-/,  at  station  A  to  ground.  The  bell  will  be  operated  by  this 
current.  The  bell  at  station  B  'will  also  receive  part  of  this 
current,  but  not  be  operated  on  account  of  its  polarity.  A 
pulsatory  current,  whose  pulsations  occur  in  the  intermissions  of 
current  through  spring,  (f,  and  of  opposite  direction,  will  flow  out 
from  spring,  q\  through  conductor,  5,  of  the  plug  circuit  to 
spring,  n  ;  but  from  this  point  a^short  circuit  is  provided  through 
the  thimble,  n\  to  the  contact-piece,  o\  of  the  plug,  and  thence 
through  the  contacts,  /3  p\  of  the  key  to  earth.  Hence  no 
signaling  current  will  reach  the  line  conductor,  2,  and  the  opera- 
tion of  the  bell  at  station  D  will  be  prevented. 

By  tracing  out  the  circuits  through  the  other  jacks  it  will  be 
found  that  in  each  case  the  spring-jack  into  which  the  plug  is 
inserted  determines  which  of  the  signals  connected  with  that  line 
shall  be  operated. 

When  the  operator  has  made  a  connection  with  any  spring- 
jack,  and  has  operated  the  signal  at  the  corresponding  station, 
the  presence  of  the  plug,  o,  in  that  spring-jack  indicates  to  her, 
during  the  existence  of  the  connection,  the  station  which  has 
been  signaled.  If  it  should  be  necessary  to  signal  the  same 
station  again,  she  does  not  have  to  remember  which  party  on 
that  line  has  been  signaled,  for  she  may  be  sure  of  again  calling 
the  same  one  by  merely  pressing  the  key,/.  If  it  should  be 
necessary  to  make  any  charge,  as  in  the  case  of  a  toll  connection, 
the  identity  of  the  station  signaled  is  ascertained  by  the  presence 
of  the  connecting  plug  in  the  corresponding  spring-jack. 

A  system  devised  by  Mr.  W.  W.  Dean,  now  of  the  Western 
Electric  Company,  and  based  on  the  same  principles  as  those  of 
Hibbard  and  McBerty,  but  adapted  for  eight  stations  instead  of 
four,  as  in  each  of  those  systems,  is  shown  in  Fig.  259.  The 
Hibbard  and  McBerty  systems  may  be  called  polarity  systems 
only,  the  strength  of  the  current  playing  no  part  in  the  selection 
of  the  particular  station  to  be  called.  The  Dean  system,  how- 
ever, is  one  of  the  few  examples  of  a  true  strength  and  polarity 
system — that  is,  one  depending  on  both  the  polarity  and  the 
strength  of  the  current.  In  this  system  four  stations  are  associ- 
ated with  each  branch  or  limb  of  a  metallic-circuit  line.  The 
two  call-bells  on  each  of  the  limbs  at  the  four  stations  farthest 
away  from  the  central  office  are  oppositely  polarized  and  bridged 
between  the  respective  line  wires  and  ground,  in  exactly  the  same 
manner  as  in  the  four-party  lines  of  Hibbard  and  McBerty.  In 
fact,  the  four  stations  at  the  farthest  end  of  the  line  from  the 


328 


AMERICAN    TELEPHONE  PRACTICE. 


central  office  may  be  considered,  so  far  as  the  signaling  is  con- 
cerned, as  the  counterpart  of  those  systems  already  described. 
The  two  call-bells  on  each  limb  at  the  four  stations  nearest  the 
central  office  are  low-wound  and  placed  in  the  line  wires.  They 
are  also  oppositely  polarized.  A  relay  is  provided  for  each 
limb,  each  having  a  high-resistance  magnet  and  being  bridged  to 
ground  at  a  point  between  the  two  high-resistance  bells  and  the 
two  low-resistance  bells  on  each  limb.  Each  of  these  relays, 


Fig-  259.— Dean  Eight-Station  Line. 

when  operated,  serves  to  ground  the  opposite  limb  of  the  line  at 
that  point. 

The  principle  of  operation  of  this  system  is  that  a  current 
adapted  to  ring  one  of  the  high-resistance  bridge  bells  at  one  of 
the  four  more  remote  stations  will  not  be  of  sufficient  strength, 
owing  to  the  high  resistance  of  the  circuit,  to  ring  one  of  the 
low-wound  series  bells  at  the  four  nearer  stations.  Therefore, 
under  ordinary  circumstances  any  one  of  the  four  stations  having 
bridged  bells  may  be  called  by  exactly  the  same  method  as 
those  described  in  connection  with  the  Hibbard  system.  When, 
however,  one  of  the  four  nearer  stations  is  to  be  called,  the  relay 
on  the  limb  to  which  the  bell  of  that  station  is  not  attached,  is 


PARTY  LINES—STRENGTH  AND  POLARITY.  329 

actuated.  This  grounds  the  limb  of  the  line  on  which  the 
desired  bell  is  placed  and  therefore  cuts  out  the  high-resistance 
bells  on  the  farther  end  of  the  line.  A  current  of  proper  polarity 
is  then  sent  over  this  limb,  which  current  is  now  capable  of  ring- 
ing the  desired  bell  on  account  of  the  low  resistance  encountered. 
This  method  of  doubling  up  the  capacity  of  a  line  by  such 
simple  means  is  characteristic  o'f  Mr.  Dean's  work  in  general. 

A  consideration  of  Fig.  259  vwill  make  clearer  the  operation 
and  details  of  this  system,  i,  2,  3,  4,  5,  6,  7,  and  8  represent  the 
subscribers'  stations  on  a  metallic  circuit,  composed  of  wires,  b 
and  c.  K  and  Kl  represent  the  calling  and  answering  plugs  re- 
spectively at  the  central  office ;  g  is  a  battery  or  other  direct-cur- 
rent generator,  while  d  is  a  generator  from  which  pulsating 
currents  of  either  positive  or  negative  polarity  may  be  taken  as 
desired  ;  /',  /2,  /3,  etc.,  are  keys  adapted  to  send  positive  or  nega- 
tive pulsating  currents,  or  direct  current  from  the  battery,  gt  over 
either  side  of  the  metallic  circuit  to  which  the  plug,  K,  is  con- 
nected by  being  inserted  in  the  spring-jack,  a. 

At  stations  I  and  2  the  positively  and  negatively  polarized 
high-resistance  bells,  el  and  e\  are  bridged  between  the  limb,  b, 
of  the  line  and  ground.  A  current  from  the  negative  side  of  the 
generator,  if  sent  over  the  limb,  b,  will  therefore  actuate  bell, 
^3,  el  not  being  actuated  on  account  of  its  not  being  responsive 
to  currents  in  that  direction.  This  current  will  also  traverse  the 
ringer  coils,  e*  and  e6,  at  stations  5  and  6,  but  will  not  operate 
them  because  too  feeble,  these  bells  being  wound  to  a  rather  low 
resistance  and  also  shunted  by  a  dead  resistance  in  order  to 
further  reduce  their  sensibility.  Suppose,  however,  station  6  is 
to  be  called ;  the  operator  first  sends  a  direct  current  from  the 
battery,  g,  over  the  line  wire,  c,  which  current  operates  the  relay, 
h,  and  causes  it  to  hold  its  contact  points  closed.  This,  as  will 
be  seen,  grounds  the  limb,  b,  at  a  point  between  the  first  high- 
resistance  bell  and  the  last  low-resistance  bell  on  that  limb.  A 
pulsating  current  from  the  negative  side  of  the  generator  is  then 
sent  over  the  limb,  b,  which  passes  through  bells,  e*  and  e',  and  to 
ground  at  the  relay,  h.  Inasmuch  as  this  current  does  not 
encounter  the  high  resistance  of  the  bell  magnets  beyond,  it  has 
sufficient  strength  to  operate  bell,  e6,  but  does  not  operate  bell, 
^8,  because  it  is  of  the  wrong  polarity.  The  selection  of  any 
station  whose  bell  is  connected  with  the  other  limb,  c,  of  the  line 
is  performed  in  exactly  the  same  manner.  The  ringing  keys, 
r  to  /8,  inclusive,  are  so  arranged  that  pressure  upon  any  one  of 
them  will  send  the  proper  current  or  currents  to  line.  For 


33°  AMERICAN    TELEPHONE   PRACTICE. 

instance,  depressing  I1  will  ground  the  negative  side  of  the 
generator  and  connect  the  positive  side  with  the  limb,  b,  which 
will  therefore  call  station  No.  I.  If  one  of  the  buttons  designed 
to  ring  the  four  nearer  stations  is  depressed,  it  will,  besides  send- 
ing the  proper  pulsating  current  to  line,  also  send  the  direct 
current  from  battery,  g,  to  the  opposite  line,  in  order  to  operate 
the  relay,  f  or  /z,  as  the  case  may  be. 

Although  not  forming  a  part  of  the  selective  signaling  system, 
the  arrangement  for  accomplishing  the  centralization  of  all 
transmitter  batteries  will  be  described,  because  it  is  of  much 
general  interest.  The  battery,  g',  is  connected  to  the  centers  of 
both  sides  of  an  induction  coil  placed  in  the  cord  circuit.  Sup- 
pose the  receiver  of  station  5  to  be  removed  from  its  hook,  the 
current  from  g'  will  proceed  to  the  center  of  the  induction  coil 
in  the  cord  circuit,  where  it  will  divide,  passing  in  parallel  over 
the  two  wires,  b  and  c,  of  the  line.  It  will  then  pass  to  the  con- 
tact points,  o  and/,  of  the  switch-hook,  and  to  the  center  point 
of  the  secondary  of  the  induction  coil  at  station  5.  Here  it  will 
again  divide,  one-half  passing  through  the  transmitter,  T,  and 
the  other  half  through  the  resistance  coil,  R,  to  the  ground  at 
G.  The  coil,  R,  has  the  same  resistance  as  the  transmitter,  T, 
under  normal  conditions.  When,  however,  the  resistance  of  the 
transmitter,  T,  is  lower,  the  greater  portion  of  the  current  will 
flow  through  it  and  a  smaller  portion  through  R,  giving  the 
equivalent  of  a  current  from  left  to  right  in  the  primary  coil,  P, 
of  the  induction  coil.  This  will  induce  a  current  in  the  ordinary 
manner  in  the  secondary,  which  will  pass  over  the  line  and  affect 
any  other  receiver  connected  with  the  circuit.  An  increase  in 
the  resistance  of  the  transmitter,  T,  will  produce  an  opposite 
result,  thus  causing  an  induced  current  in  the  opposite  direction 
to  flow  in  the  line.  Thus  while  the  current  from  battery,  g't 
produces  no  effect  on  the  apparatus  in  the  line  under  ordinary 
circumstances,  it  supplies  the  current  for  the  local  circuit  of  a 
station  which,  when  operated  upon  by  the  transmitter,  affects 
inductively  the  secondary  circuit  connected  with  the  line. 

A  system  which  is  being  put  into  practical  operation,  and  is 
apparently  meeting  with  much  success,  was  recently  devised  by 
Messrs.Barrett,  Whittemore  and  Craft.  It  depends  for  its  operation 
on  the  sending  of  currents  of  either  polarity,  or  different  combina- 
tions of  currents,  over  either  or  both  of  two  line  wires  in  combi- 
nation with  each  other  or  with  the  ground.  Thus  calling  one  line 
wire  A,  and  the  other  B,  and  representing  the  ground  by  G,  it  is 
evident  that  without  using  wire,  B,  at  all,  a  current  could  be  sent 


PARTY  LINES— STRENGTH  AND  POLARITY. 


331 


over  wire,  A,  with  a  ground  return  in  either  direction,  thus  giv- 
ing means  for  two  selective  signals.  Similarly  leaving  A  out  of 
the  question,  a  current  of  either  direction  could  be  sent  over  B, 
with  a  ground  return,  thus  providing  for  two  other  selective 
signals.  So  far  the  combinations  are  identical  with  those  of 
Hibbard.  A  current  may  also  be  sent  in  either  direction  over 
the  metallic  circuit  formed  by  A  and  B,  thus  providing  for  two 
other  signals  ;  and  lastly,  by  using  A  and  B  in  multiple,  currents 
could  be  sent  in  either  direction,  using  a  ground  return,  thus 
affording  means  for  two  more  signals,  or  eight  in  all.  Two  other 
combinations  might  be  obtained  by  sending  currents  in  either 
direction  over  wire,  A,  and  using  wire,  B,  and  the  ground  in 
multiple,  as  a  return  ;  and  similarly  two  others  by  using  B  for 
one  side  of  the  circuit  with  the  wire,  A,  and  ground  in  multiple 

CURRENT  COMBINATIONS. 


Line  A. 

Line  B. 

Ground. 

I  

-j- 

o 

2 

o 

-j- 

o 

-j- 

4   

o 

-f- 

c 

1 

o 

6 

_l_ 

o 

7.  ......                     .               ..... 

-(- 

_l_ 

8  

-)- 

for  a  return.  These  latter  combinations,  however,  have  been 
found  to  introduce  undesirable  features,  as  will  be  understood 
later  on.  The  eight  desirable  current  combinations  may  be  tabu- 
lated as  in  the  table  above. 

In  this  table  the  plus  and  minus  signs  indicate  which  pole  of 
the  calling  battery  at  central  is  connected  to  either  line  wire  or 
ground.  Thus,  in  the  first  combination,  the  positive  pole  is  con- 
nected with  line,  A,  the  negative  with  the  ground  in  order  to 
utilize  the  earth  return.  Line,  B,  in  this  combination,  is  not  used 
at  all. 

Fig.  260  shows  diagrammatically  such  an  arrangement  of 
apparatus  at  eight  stations  that  the  call-bell,  D,  at  each  station 
will  be  actuated  only  when  the  one  particular  set  of  current 
combinations  is  sent  over  the  line.  A  and  B  represent  two  line 
wires  extending  from  a  central  station,  C,  to  a  number  of  sub- 
stations. 5,  S2,  S3,  etc.  At  each  of  the  substations  are  two  relays, 
R  and  R\  placed  in  earth  branches,  m  and  q,  from  the  two  line 


332 


AMERICAN    TELEPHONE   PRACTICE. 


wires,  A  and  B,  respectively.  These  two  branches  are  united  at  ey 
and  connected  with  the  ground  at  G.  The  signal  bell,  /?,  is  con- 
nected with  the  local  battery,  s,  in  a  circuit,  the  continuity  of 
which  is  controlled  by  each  of  the  relays,  R  and  R*.  Unless  the 
armatures,  13,  of  both  relays  rest  against  their  back  stops,  12,  the 
local  circuit  containing  the  bell  will  be  opened  at  one  or  two 
points.  The  relays  of  each  station  differ  in  some  way,  either  in 
construction  or  arrangement,  from  those  of  all  other  stations. 
Thus  at  station,  5,  the  main  conductor,  A,  is  branched  through  a 


»* 


•r 

k 


f 


X 

y 


Fig.  260.— Simplified  Barrett-Whittemore-Craft  System. 

polarized  relay  made  responsive  to  positive  currents  from  the 
central  office,  and  the  main  conductor,  B,  through  a  neutral  relay, 
R\  adapted  to  respond  to  currents  of  either  direction  from  the 
central  office.  It  is  thus  obvious  that  if  a  positive  current  is 
sent  over  wire,  A,  without  sending  any  current  whatever  over 
B,  the  bell  at  station,  S,  will  be  operated  because  the  posi- 
tive current  will  cause  the  relay,  R,  to  release  its  armature, 
while  the  armature  of  relay,  R\  is  already  released.  Thus,  both 
contacts,  10  and  n,  will  be  closed  and  the  bell  circuit  complete. 
Station,  5a,  also  has  a  neutral  relay  on  wire,  B,  and  a  negatively 
polarized  relay  on  wire,  A.  The  third  and  fourth  stations,  S\ 


PARTY  LINES— STRENGTH  AND   POLARITY.  333 

and  S\  each  have  a  neutral  relay  on  wire,  Ay  and  a  positively  or 
negatively  polarized  relay  on  wire,  B.  The  fifth  station,  S\  has 
two  polarized  relays,  one  adapted  to  respond  to  positive  currents 
and  attached  to  wire,  A,  and  the  other  to  negative  currents  and 
attached  to  wire,  B.  The  sixth  station,  S6,  also  has  oppositely 
polarized  relays,  but  their  connection  with  the  line  is  the  reverse 
of  that  in  station,  Sb.  The  seventh  station,  S7,  has  two  positive 
relays  and  the  eighth  station,  5 ,  two  negative  relays,  one  in  each 
case  being  bridged  between  each  limb  of  the  line  and  ground. 

Reference  to  the  table  of  current  combinations  will  show,  in 
connection  with  Fig.  260,  that  the  sending  of  any  particular 
combination  to  line  will  operate  the  relays  of  the  station  bearing 
the  corresponding  number  in  such  manner  as  to  close  the  local 
circuit  at  that  station.  Further  consideration  will  also  show  that 
no  combination  will  so  operate  the  relays  at  more  than  one 
station. 

At  the  central  station,  B',  is  a  generator  of  calling  current,  and 
G',  an  earth  connection  complementary  to  the  earth  connections, 
G,  at  the  substations.  K  is  a  group  of  signaling  keys,  each  cor- 
responding with  one  substation  appliance,  and  when  any  partic- 
ular key  is  pressed  it  sends  the  proper  current  combination  to 
line  so  that  the  relays  at  the  particular  substation  represented  by 
it  will  co-operate  to  close  the  local  circuit  and  give  the  signal 
there ;  but  at  the  other  stations  no  such  effect  will  take  place. 
Hence,  to  give  a  signal  at  any  desired  substation,  it  is  only 
necessary  to  operate  the  particular  key  representing  such  station. 
To  accomplish  this,  branch  terminals  are  brought  from  the  line 
conductors,  A  and  B,  from  the  ground  connection,  G',  and  from 
the  positive  and  negative  poles  of  the  battery  to  the  various 
terminals  on  the  signaling  keys.  The  arrangement  of  the  ter- 
minal contacts  in  each  key  is  different,  the  differences  correspond- 
ing with  those  of  the  substation  relay  arrangement. 

To  illustrate :  in  key  No.  I  the  contacts  are  so  disposed  that 
its  operation  will  connect  conductor,  A,  with  the  positive  pole  of 
the  battery,  B',  at  contacts,  v  and  j/,  the  minus  pole  of  the 
generator  with  the  earth  terminal-contacts,  z  and  w,  and  will 
leave  conductor,  B,  disconnected.  By  this  means  a  positive  cur- 
rent is  sent  over  line,  A,  and  is  distributed  through  all  the  A 
relays  at  all  of  the  substations  in  parallel,  finding  return  through 
the  earth  branches;  but  as  no  current  is  transmitted  over  line 
conductor,  B,  all  of  the  eight  B  relays  will  remain  unaffected. 
Under  these  conditions  relay,  R,  at  station,  5,  will  close  point,  10, 
of  its  local  circuit,  and  the  point,  II,  being  already  closed  by  the 


334  AMERICAN    TELEPHONE   PRACTICE. 

armature  of  relay,  R*,  the  normal  position  of  which  has  not  been 
changed,  the  local  circuit,  c,  of  station,  5,  will  be  closed  and  the 
bell  at  this  station  will  be  rung.  Station,  .S2,  will  not  be  signaled, 
because  plus  currents  have  no  effect  on  its  polar  relay,  R. 
Station,  S3,  is  not  signaled,  because  the  effect  of  the  plus  current 
on  main,  A,  is  to  attract  the  armature  of  neutral  relay,./?,  and  thus 
open  the  local  circuit,  which  is  already  open  at  point,  1 1.  Station, 
S\  receives  no  signal  for  the  same  reason.  Station,  S&,  is  not 
signaled,  because,  though  the  positively  polarized  relay  on  A 
closes  the  open  point,  10,  of  its  local  circuit,  the  said  circuit 
remains  open  at  n,  there  being  no  current  on  B ;  station,  56, 
because  neither  relay  is  acted  upon,  R  being  of  minus  polarity 
and  R*  having  no  current ;  station,  S7,  because  R  alone  is  operated, 
and  station,  S8,  because  both  relays  are  of  minus  polarity. 

In  applying  the  principles  already  pointed  out  to  a  practical 
multiple-station  circuit,  it  is  desirable  to  reserve  two  of  the  current 
combinations  for  the  operation  of  locking  devices  common  to  all 
stations. 

The  seventh  and  eighth  combinations  in  the  foregoing  table 
have  been  found  most  convenient  for  this  purpose.  The  seventh, 
that  is,  the  positive  current  over  both  conductors,  A  and  B,  in 
parallel,  is  used  for  locking  the  telephone  apparatus  at  all  sta- 
tions, and  a  negative  current  over  both  lines  for  unlocking  the 
apparatus.  Six  combinations  are  thus  left  for  signaling. 

The  locking  device  and  a  visual  busy  signal  are  shown  in  asso- 
ciation with  complete  telephone  equipments  at  two  stations  in 
Fig.  261.  In  these  an  additional  electromagnetic  apparatus,  R3, 
is  shown  in  circuit  with  the  relays,  R  and  R\  at  each  substation, 
half  of  its  winding  being  in  the  earth  branch,  m,  of  the  relay,  R, 
and  half  in  the  branch,  q,  of  the  relay,  R*. 

Two  electromagnetic  helices,  a  and  b,  have  the  ends  of  their 
cores  joined  by  soft-iron  yoke-pieces  to  form  the  instrument,^3. 
Two  soft-iron  polar  extensions,  //  and  f,  project  inwardly  from 
the  yoke-pieces  as  shown.  A  polarized  bar  armature,  /,  pivoted  at 
y2,  has  one  of  its  poles  projecting  between  the  pole-pieces,  h  and 
/,  and  adapted  to  move  to  one  side  or  the  other  under  the  in- 
fluences of  said  pole-pieces.  If  current  is  passed  through  coil,  a, 
only,  the  magnetic  polarity  developed  will  be  short-circuited 
through  the  yoke-pieces  and  the  core  of  coil,  b,  so  that  very  little 
strength  will  be  manifested  in  the  pole-pieces,  h  and/;  if  current 
be  applied  to  the  coil,  b,  only,  the  magnetic  polarity  will  be  simi- 
larly short-circuited,  and,  again,  little  effect  will  be  manifested  in 
the  pole-pieces.  Again,  if  current  be  applied  to  both  coils,  a  and 


PARTY  LINES— STRENGTH  AND  POLARITY. 


335 


b,  so  as  to  act  in  a  complementary  direction,  the  yoke-pieces  will 
satisfy  the  magnetic  flux  with  very  little  polarity  in,  h  and/;  but 
if  current  be  applied  to  coils,  a  and  b,  in  inductively  opposed  di- 
rection, as  will  be  the  case  when  the  seventh  and  eighth  combi- 
nations are  transmitted,  consequent  poles  of  full  strength  and 


Fig.  261. — Lock-out  Mechanism. 

opposite  polarity  will  be  formed  at  h  and/.  The  polarized  lever, 
/,  is,  therefore,  actuated  by  the  seventh  and  eighth  current  com- 
binations and  remains  unaffected  by  all  others. 

As  shown  at  the  right  of  Fig.  261,  the  lever,/,  serves  not  only 
as  a  lockout  device,  but  also  as  a  busy  signal.  The  apparatus 
is  shown  in  its  locked  or  busy  position  at  station,  S2,  of  this  figure 
and  in  its  unlocked  or  free  position  in  station,  S3.  When  the 
lower  portion  of  the  lever  is  moved  to  the  left  it  forms  a  stop  to 


336 


AMERICAN   TELEPHONE  PRACTICE. 


lug,/*,  on  the  hook-switch,  z,  and  thus  prevents  the  latter  from 
rising  should  the  receiver  be  removed  from  the  hook.  At  the 
same  time  the  small  target,  B,  on  the  other  end  of  the  lever  is  dis- 
played through  a  hole  in  the  box,  thus  showing  the  party  at  that 
station  that  the  line  is  busy.  When  in  its  other  position  the 
busy  signal  is  not  displayed  and  the  hook-switch  is  free  to  rise. 


Fig.  262.— Circuits  of  Six-Station  B.  W.  C.  System. 

When  the  operator  at  central  presses  the  locking  key,  say  key 
No.  7,  all  of  the  locking  levers  on  the  line,  including  that  of  the 
party  to  be  called,  will  be  actuated.  In  order  that  the  party 
being  called  may  not  be  thus  locked  out,  the  windings,  27  and  28, 
are  provided  around  the  polar  extensions,  h  and  f,  on  each  in- 
strument. This  winding  has  no  function  except  at  the  station 
being  called.  In  that  station  part  of  the  current  from  the  local 
circuit,  which  is  closed  only  at  that  station  by  the  action  of  the 
relays,  finds  path  through  this  winding,  and  the  magnetism  so 
developed  serves  to  unlock  the  mechanism  and  to  allow  the  .party 
at  that  station  to  use  his  instrument. 


PARTY  LINES— STRENGTH  AND  POLARITY.  337 

In  Fig.  262  is  shown  a  six-party  line,  the  equipment  at  each 
station  being  of  a  similar  character  to  that  shown  in  Fig.  261,  but 
simplified  for  the  purpose  of  clearer  illustration.  The  two  sides 
of  the  line  terminate  in  the  line  springs  of  a  spring-jack,/,  which 
springs  normally  rest  on  anvils  connected  to  the  windings,  31  and 
32,  of  a  differentially  wound  switch-board  drop.  These  two  wind- 
ings pass  around  the  core  of  the  drop  magnet  in  opposite  direc- 
tions, after  which  they  unite  at  the  point,  60,  and  pass  to  ground1 
through  a  battery,  B?.  The  relative  direction  of  the  windings  on 
the  drop  is  such  that  the  current  from  this  battery  circulates 
around  the  core  in  opposite  directions,  and  thus  does  not  affect 
the  drop.  It  then  divides  equally  between  the  two  main  con- 
ductors, A  and  B,  and  finally  returns  by  the  ground  connections,. 
(7,  at  each  of  the  several  stations.  The  current  thus  flowing  to 
the  two  conductors  from  the  battery,  B\  is  in  a  negative  direc- 
tion, and  thus  tends  to  maintain  the  apparatus  at  the  several 
stations  in  its  unlocked  condition. 

When  any  subscriber  removes  his  receiver  from  the  hook,  the 
short  arm  of  the  hook-lever,  L,  makes  contact  momentarily  with 
the  spring,  d,  which  grounds  the  main  line  wire,^,  and  thus  allows 
a  heavy  current  to  pass  through  the  winding,  32,  of  the  drop,  A 
This  throws  the  drop  and  attracts  the  attention  of  the  operator. 
The  operator  answers  the  call  in  the  ordinary  way  by  the  inser- 
tion of  one  of  the  plugs,  P,  with  which  the  ringing  keys,  /,  in  Fig. 
260  are  associated. 

When  a  substation  is  to  be  signaled,  the  calling  plug,  P,  is 
inserted  into  the  spring-jack,  which  cuts  off  the  annunciator 
and  connects  the  keys,  K,  with  that  particular  circuit.  Key,  l\ 
which  sends  the  plus  current  over  both  mains  in  parallel,  is 
then  operated  to  lock  the  apparatus  at  all  stations  without 
ringing  any  of  the  bells  ;  and  then  the  key  representing  the  de- 
sired station  is  pressed  which  results  in  ringing  the  bell,  and  at 
the  same  time  in  releasing  the  telephone  apparatus  at  that  station 
by  the  means  already  described.  At  the  close  of  any  conversa- 
tion the  key,  /8,  sending  a  negative  current  over  both  mains  in 
parallel,  is  operated  to  release  the  apparatus  at  all  stations,  re- 
storing the  circuit  to  its  normal  condition. 


CHAPTER  XXVIII. 

PARTY   LINES. — HARMONIC    SYSTEMS   OF   SELECTIVE   SIGNALING. 

THE  third  general  method  of  selective  signaling  on  party  lines 
makes  use  of  the  fact  that  every  pendulum  or  elastic  reed  has  a 
natural  period  of  vibration,  and  that  it  can  be  made  to  take  up 
this  vibration  by  the  action  of  a  succession  of  impulses  of  force 
occurring  in  the  same  period  as  that  of  the  reed  or  pendulum. 
A  familiar  example  of  this  is  found  in  one  person  pushing 
another  in  a  swing.  The  swing  has  its  natural  period  of  vibra- 
tion, depending  on  the  length  of  the  ropes,  and  a  gentle  push 
applied  at  proper  intervals  by  the  person  on  the  ground  will 
cause  the  swing  to  vibrate  with  a  considerable  amplitude.  If  the 
pushes  are  applied  at  intervals  not  corresponding  to  the  natural 
period  of  vibration  of  the  swing,  many  of  them  tend  to  retard 
rather  than  help  its  vibrations,  so  that  a  useless  bumping  results, 
which  produces  but  little- motion. 

The  utilization  of  this  principle  has  given  inventors  a  very 
attractive  field  of  work  ;  but  as  in  the  case  of  the  step-by-step 
systems,  the  results  attained  have  been  of  little  practical  value  in 
telephony,  except  in  so  far  as  they  have  contributed  to  the 
fgeneral  stock  of  knowledge  on  the  subject. 

The  idea  of  selective  signaling  between  different  instruments 
in  the  same  circuit  was  used  in  telegraphy  before  the  birth  of 
telephony.  A  number  of  currents  of  different  rates  of  vibration 
were  impressed  upon  the  circuit  by  as  many  different  transmitters, 
each  particular  rate  of  vibration  being  capable  of  operating  a 
reed  in  one  of  the  receiving  instruments,  and  producing  no  effect 
upon  the  others.  By  this  means  each  receiving  instrument  was 
capable  of  picking  out  only  those  signals  sent  by  the  transmitter 
having  the  same  rate  of  vibration,  and  thus  all  of  the  transmitters 
could  be  used  simultaneously  in  the  same  circuit,  producing  a 
system  of  multiplex  telegraphy. 

The  idea  as  applied  to  telephony  is  shown  in  Fig.  263,  where 
C  is  an  electromagnet  connected  with  the  line  wire,  A  A',  in 
serjes  with  similar  magnets  at  all  of  the  other  stations.  B  is  an 
armature  of  soft  iron  mounted  on  the  post,  by  by  a  short  flat 
spring,  thus  forming  a  reed  which  it  is  obvious  will  have  a  fixed 

338 


PARTY  LINES— HARMONIC  SYSTEMS. 


339 


rate  of  vibration  for  any  particular  adjustment.  When  a  number 
of  current  impulses  are  sent  over  the  line  wire  having  a  frequency 
corresponding  to  the  rate  of  vibration  of  the  reed,  B,  the  latter 
will  be  thrown  into  vibration.  If  the  frequency  of  the  current 
impulses  does  not  correspond  to  the  rate  of  vibration  of  the 
reed,  then  the  reed  will  vibrate  but  slightly,  if  at  all.  D  is  a 


Fig-.  263. — Currier  and  Rice  Harmonic  Selector. 

flexible  spring  forming  a  part  of  a  secondary  circuit  containing 
an  ordinary  vibrating  bell.  When  the  reed,  B,  is  thrown  into  a 
sufficiently  wide  vibration,  this  latter  circuit  is  closed  at  the  point, 
n,  and  the  bell  is  sounded. 

The  reeds  at  all  of  the  stations  are  so  adjusted  as  to  have 
different  rates  of  vibration,  and  by  impressing  current  impulses 
of  a  proper  frequency  upon  the  line  at  the  central  station,  the 
bell  at  the  desired  station  can  be  sounded.  This  illustration  is 
that  of  a  device  invented  by  Messrs.  Currier  and  Rice  in  1880. 

Fig.  264  shows  a  signal-receiving  instrument  designed  three 
years  later  by  Elisha  Gray  and  Frank  L.  Pope.  M  is  an  electro- 
magnet having  polar  extensions,  m  and  m'  (best  shown  in  the 
plan  view),  between  which  is  pivoted  the  polarized  armature,  P. 
This  will  be  attracted  toward  one  or  the  other  of  the  polar 
extensions,  according  to  the  direction  of  the  current  through  the 
coils.  O  is  a  vibrating  reed  having  one  end  rigidly  mounted  on 
the  post,  O'.  The  rate  of  vibration  of  this  reed  may  be  varied 


340 


AMERICAN    TELEPHONE   PRACTICE. 


by  the  sliding  weight,  o,  which  may  be  clamped  in  any  desired 
position  by  the  tnumbscrew,  o'.  N  is  an  armature  by  which  the 
electromagnet,  M,  may  exert  its  influence  on  the  reed,  O.  R  is 
a  separate  lever  pivoted  as  shown  and  normally  making  contact 
with  the  reed. 

Three  such  receiving  instruments  are  shown  connected  in  a 
line  circuit,  L,  in  Fig.  265.  At  the  top  portion  of  this  figure  is 
shown  the  transmitting  apparatus  at  the  central  office.  The 
three  transmitters,  B\  B*y  B*,  have  each  a  vibrating  reed,  b,  play- 
ing between  two  pairs  of  electromagnets,  E  and  e,  and  main- 


Fig.  264. — Gray  and  Pope  Harmonic  Mechanism. 

tained  in  constant  vibration  by  the  alternate  passage  through 
these  magnets  of  currents  from  the  local  batteries,  F.  The 
reed  of  each  transmitter  is  attuned  to  the  rate  of  vibration  of 
the  reed  of  the  corresponding  receiver,  and  therefore  current 
impulses  sent  to  line  from  any  transmitter  will  operate  only  one 
of  the  receivers. 

A  constant  current  is  maintained  upon  the  main  line,  L,  by 
means  of  a  main  battery,  G,  at  the  central  office.  When  the 
apparatus  is  at  rest,  the  circuit  may  be  traced  as  follows:  from 
the  earth  at  the  central  office  by  the  wires,  w  and  zv\  to  the 
contact  point,  v\  thence  by  the  contact-springs,  s',  to  the  contact- 
stop,  v\  contact-spring,  /,  stop,  ^3,  contact-spring,  s\  wires,  w*  w3, 
and  contact-stop,  w*,  to  the  contact-spring,  x,  and  thence  by  the 
wire,  ze/4,  to  the  positive  pole  of  battery,  G;  thence  from  the  negative 
pole  by  the  wire,  w\  to  the  contact-spring,  xl,  thence  by  contact- 
stop,  w\  and  wires,  w"  w\  to  the  electromagnet  of  the  signal 
bell,  D,  and  thence  to  the  line,  L,  which  extends  to  and  through 
the  several  stations,  and  finally  to  earth  at  the  terminal  station. 

Upon  an  insulating  support,  T,  is  mounted  a  series  of  metallic 
springs,  t\  f,  and  t\  carrying  buttons,  c\  c\  and  c\  the  free  ends 
of  which  springs  project  over  the  free  ends  of  the  series  of  con- 


PARTY  LINES— HARMONIC  SYSTEMS. 


341 


tact-springs,  s1,  s*,  s3.  The  contact-springs,  x  x\  are  also  mounted 
upon  the  insulating  support,  T,  their  free  ends  being  united  by 
a  non-conducting  bar,  X,  which  passes  directly  underneath  the 
free  ends  of  the  springs,  s1,  J2,  and  s\  The  key  springs,  t\  f,  f, 
are  connected  by  wires,  yl,  y1,  y*,  with  the  respective  reeds  of  the 


Fig.  265. — Gray  and  Pope  Party  Line. 

transmitters,  Bl,  £*,  B*,  after  which  they  are  united  to  a  common 
wire,  z,  which  is  connected  directly  with  the  earth. 

At  each  substation  is  placed  the  receiver  already  described,  a 
key,  H,  a  battery,  <2,  and  a  vibrating  bell,  J.  The  polarized 
armature,  P,  of  the  receiver  is  held  in  such  a  position  by  the 


342  AMERICAN    l^ELEPHONE   PRACTICE. 

normal  current  from  the  battery,  G,  at  central  as  to  hold  the 
local  circuit  open  at  the  point,  q\  Besides  this,  a  shunt  is 
normally  closed  around  the  bell  magnet,  K,  at  each  station,  by 
the  closure  of  the  contact  between  the  reed,  N,  and  the  arm,  R. 

To  call  central  the  party  at  a  substation  has  only  to  depress 
his  key,  H.  This  breaks  the  line  circuit  and  allows  the  hammer 
of  the  central-office  bell,  D,  to  strike  one  blow.  When  the 
operator  at  the  central  office  wishes  to  transmit  a  call  to  one  of 
the  substations — for  example,  station  2 — she  depresses  the  key, 
C*.  This  establishes  a  connection  between  the  springs,  f  and  s\ 
and  at  the  same  time  breaks  the  contact  previously  existing 
between  the  spring,  ja,  and  the  stop,  V*.  By  the  same  operation 
the  bar,  X,  is  depressed  and  the  springs,  x  xl,  are  respectively 
removed  from  contact  with  the  terminals,  w9j  and  u>g,  and  brought 
into  contact  with  the  wires,  w1  and  w1 .  This  operation  produces 
the  twofold  effect  of  switching  the  main-line  circuit  through  the 
appropriate  vibrating  transmitter  reed,  B? ',  and  of  reversing 
the  polarity  of  the  main  battery,  G,  with  respect  to  the  line. 
The  change  of  the  polarity  of  the  main-line  current  causes  the 
polarized  armature,  P,  at  every  substation  to  be  deflected  from 
its  normal  position,  thus  bringing  it  in  contact  with  the  stop,  q\ 
and  closing  the  circuit  of  the  local  battery,  Q.  The  closing  of 
the  local  battery  in  this  manner  will,  however,  in  itself  produce 
no  effect  upon  the  electromagnet,  K,  of  the  bell,  as  the  latter  is 
shunted  by  the  contact  between  the  reed,  O,  and  the  bar,  R, 
which  rests  upon  it.  The  reed,  O,  at  station  2  being  adjusted 
to  vibrate  in  response  to  the  reed  of  the  transmitter,  j52,  will  be 
set  in  vibration,  and  this  vibration  will  cause  the  loosely  pivoted 
bar,  R,  to  hop  up  and  down,  interrupt  the  shunt-circuit,  and 
allow  the  magnet,  K,  to  become  active,  thus  causing  the  bell,  /, 
to  ring.  The  bells  at  all  the  other  substations,  being  cut  out  by 
the  action  of  their  respective  shunts,  will  remain  quiescent. 
The  bell  of  any  other  station  is  actuated  in  precisely  the  same 
manner,  the  only  difference  being  that  the  reed-armature,  O,  in 
each  instance  is  adjusted  to  vibrate  in  harmony  with  its  corre- 
sponding transmitter  at  the  central  office  and  to  respond  only  to 
currents  sent  to  line  by  it. 

The  device  of  Currier  and  Rice  depended  on  the  vibrating 
reed  to  close  the  circuit  through  the  call-bell,  while  in  that  of 
Gray  and  Pope  the  reed  served  only  to  break  a  shunt  around  the 
belL  In  Fig.  266  is  shown  a  system  designed  by  J.  A.  Light- 
hipe  of  San  Francisco,  in  which  the  gongs  are  struck  directly  by 
the  reed,  without  the  use  of  an  auxiliary  magnet.  This  will  be 


PARTY  LINES— HARMONIC  SYSTEMS. 


343 


understood  from  the  diagram  without  much  explanation.  The 
reeds,  <^2,  e*,  k?,  and  /2,  carry  hammers  adapted  to  play  between 
the  gongs  at  the  substations  when  acted  upon  by  their  magnets, 
d,  e,  k,  or  /.  At  stations,  A  and  B,  these  magnets  are  bridged 
directly  across  the  two  sides  of  the  metallic  line,  while  at  stations, 
C  and  D,  on  another  line,  they  are  bridged  between  one  side  of 
the  circuit  and  ground.  A  condenser,  d'  or  e',  is  in  each  bridge 
wire  in  the  former  case,  to  prevent  the  leakage  of  current  from 


j> 


Fig.  266. — Lighthipe  Bridged  Harmonic  System. 

the  signaling  battery,  b',  when  the  telephones  are  not  in  use. 
Associated  with  the  cord  circuit  of  a  pair  of  plugs  at  the  central 
office  are  the  signal  transmitters,  each  having  a  reed  tuned  to 
the  rates  of  vibration  of  the  several  reeds  of  the  call  receivers. 
Pressure  of  the  button,  //,  for  instance,  closes  the  circuit  of  bat- 
tery, g,  through  electromagnet,  /^,  over  the  limb,  a,  of  the  tele- 
phone line  through  the  electromagnets,  d  and  e,  at  the  substations 
and  back  by  the  limb,  a',  to  the  opposite  pole  of  the  battery. 

The  magnet,  #*,  is  thus  excited,  and  attracts  the  reed,  which  in 
its  forward  movement  completes  a  short  circuit  around  the 
battery.  The  reed  vibrates  back  and  forth,  sending  current 


344 


AMERICAN   TELEPHONE   PRACTICE. 


impulses  over  the  circuit,  and  as  its  rate  of  vibration  is  the  same 
as  that  of  reed,  d*,  at  station  A,  these  impulses  will  have  the 
proper  frequency  to  actuate  that  reed  and  sound  its  bells.  Call- 
ing central  from  the  subscribers'  stations  is  performed  in  precisely 
the  same  way  as  in  the  Sabin  and  Hampton  and  other  systems 
already  described. 

As  the  apparatus  in  this  system  is  arranged  on  the  bridging 
plan,  it   is  of   course    necessary  that   the    bell    magnets   should 


?  a 


Fig.  267. — Harter  Harmonic  System. 

possess  high  impedance  in  conformity  with  the  requirements  of 
bridged  lines. 

Fig.  267  shows  a  somewhat  elaborate  system,  invented  by  Mr. 
William  H.  Harter,  of  Great  Falls,  Mont.,  and  embodying  a 
lockout  mechanism  in  addition  to  the  signaling  devices.  In  this 
figure  two  substations,  I  and  2,  are  shown  connected  by  a  metallic 
circuit  line,  with  two  transmitting  devices  at  the  central  office. 
Instead  of  this  connection  being  permanently  made  as  shown,  it 
would  be  brought  about  in  practice  by  a  spring-jack  and  plugs,  the 
transmitting  devices  being  connected  across  the  cord  circuit. 

The  reeds,  b  and  b\  at  the  substations,  I  and  2,  are  tuned  to  the 


PARTY  LINES— HARMONIC  SYSTEMS.  345 

same  pitch  as  reeds,  a  and  a1,  of  their  respective  transmitters  at 
central. 

Upon  connecting  the  cord  circuit  with  the  circuit  of  the  line, 
the  battery,  C,  is  thrown  across  the  two  sides  of  the  line,  and 
current  therefrom  passes  through  each  of  the  locking  magnets,  v, 
in  multiple,  attracting  their  armatures,  s,  and  locking  all  of  the 
receiver  hooks  in  their  depressed  position.  Pressure  of  key  No. 
I  (for  the  purpose  of  calling  station  No.  i)  establishes  a  local 
circuit  through  transmitter  magnet,^1,  and  the  back  contact  of  its 
reed,  thus  throwing  it  into  rapid  vibration.  By  means  of  its 
front  contact,  e,  and  the  reed,  a,  impulses  of  current  from  C  are 
allowed  to  flow  over  the  line  circuit,  through  the  magnets,  B, 
of  the  substations ;  and  as  these  are  of  the  right  frequency  to 
actuate  the  reed  at  station  No.  I,  this  reed  is  thrown  into 
vibration,  the  others  remaining  at  rest. 

The  reed,  b,  in  its  vibration  completes  a  local  circuit  contain- 
ing a  magnet,  /,  and  local  battery,  K,  and  causes  it  to  attract  its 
armature,  m, against  three  contacts,  n,  w,  and/.  The  circuit  closed 
at  the  contact,  n,  allows  the  impulses  of  current  coming  over  the 
line  from  the  battery,  C,  to  operate  the  bell,  o.  The  circuit 
closed  at  the  contact,/,  includes  also  the  contact,  n,  and  contains 
the  magnet,  /,  and  local  battery,  k,  and  thus  serves  to  keep  the 
armature,  m,  depressed,  regardless  of  the  action  of  the  reed. 
The  circuit  closed  at  the  contact,  w,  short-circuits  the  locking 
magnet,  vy  thus  releasing  the  hook-lever  at  the  station  being 
called. 

It  will  be  seen  that  the  act  of  plugging-in  locks  all  stations,  and 
the  closure  of  key  No.  I  throws  reed,  bt  at  station  No.  I  into 
vibration.  This  operates  magnet,  /,  which  closes  the  bell  circuit 
and  also  unlocks  the  hook-lever  at  that  station. 

In  practice  a  modification  of  the  central-office  circuits  would 
be  necessary,  for,  as  shown,  the  contact  made  between  the 
vibrating  reed  at  key  No.  I  and  its  contact,  e,  simply  closes 
a  circuit  from  the  battery,  6",  which  is  already  made  at  key  No.  2. 
Each  key  should,  therefore,  be  disassociated  from  the  other 
keys  during  the  transmission  of  the  vibratory  currents. 

These  are  only  a  few  of  a  large  number  of  systems  depending 
on  the  general  principles  outlined.  The  harmonic  idea  is 
attractive,  and  may  be  applied  in  a  great  number  of  ways  to  the 
solution  of  the  problem.  It  has,  however,  as  before  stated,  been 
productive  of  but  few  practical  results.  In  fact,  but  one  harmonic 
selective  signaling  system  is,  so  far  as  the  writer  is  aware,  in 
practical  operation  in  the  United  States.  It  is  in  use  by  the 


346  AMERICAN   TELEPHONE   PRACTICE. 

local  Bell  Company  at  Sacramento,  Cal.,  and  is  not  an  unqualified 
success,  although  it  has  been  used  over  three  years.  This 
slight  use  of  the  harmonic  principle  should  not  detract,  however, 
from  the  interest  in  the  subject,  for  a  knowledge  of  the 
experience  of  others  is  a  valuable  aid  in  any  branch  of  work,  and 
in  none  more  so  than  in  telephony. 


CHAPTER    XXIX. 

WIRE   FOR   TELEPHONE    USE. 

THE  wires  in  use  in  telephone  work  are,  at  present,  of  copper 
and  iron  exclusively.  Aluminum  will  probably,  as  the  price  of 
its  manufacture  is  cheapened,  come  into  extensive  use,  and  it 
will  not  be  surprising  if  it  eventually  supersedes  both  copper 
and  iron  for  all  except  very  long  distance  service.  Iron  pos- 
sesses a  slight  advantage  over  copper  on  account  of  its  tensile 
strength,  and  a  very  decided  advantage  in  point  of  first  cost,  but 
in  all  other  respects  copper  is  vastly  superior. 

The  tensile  strength  of  a  wire  is  its  ability  to  resist  a  pulling 
stress  and  the  amount  of  tensile  strength  is  usually  expressed  in 
the  number  of  pounds  necessary  to  break  a  given  wire.  The 
breaking  stress  varies,  of  course,  in  the  same  metal  with  the  size 
of  the  wire,  that  is,  with  the  area  of  its  cross-section.  The 
weight  of  a  given  wire  varies  also  in  the  same  ratio,  and  therefore, 
in  order  to  have  a  convenient  method  for  designating  the  break- 
ing strength  applicable  alike  to  all  sizes  of  wire  of  a  certain  grade, 
the  breaking  stress  is  frequently  expressed  in  the  number  of 
times  the  weight  per  mile  of  the  given  wire  necessary  to  break  it. 

Thus,  knowing  that  a  certain  grade  of  wire  has  a  breaking 
strength  equal  to  two  and  one-half  times  its  weight  per  mile,  all 
that  we  have  to  find  out  in  order  to  know  the  breaking  strength 
of  any  size  of  this  same  grade,  is  the  weight  per  mile  of  that  size. 
For  example,  a  No.  12  iron  wire  weighs  165  pounds  per  mile. 
This  we  find  out  by  consulting  any  table  giving  the  weight  of 
wire,  or  by  weighing  a  known  length  of  wire.  Knowing  that  the 
breaking  strength  of  this  grade  of  wire  is  2\  times  its  weight  per 
mile,  we  may  at  once  arrive  at  the  conclusion  that  the  breaking 
strength  of  this  particular  size  is  2£  times  165=412^  pounds. 

The  most  important  electrical  property  of  line  wire  is  its 
conductivity  per  unit  area  of  cross-section.  A  conductor  of  iron 
may  be  made  to  have  a  resistance  as  low  as  that  of  a  copper 
conductor,  by  giving  it  about  seven  times  the  cross-sectional  area. 
In  doing  this,  however,  we  make  its  inductive  capacity  much 
greater,  and,  as  has  been  shown,  this  is  a  decided  disadvantage. 
Besides  this,  the  greater  weight  of  an  iron  wire  of  the  same 


348  AMERICAN    TELEPHONE  PRACTICE. 

conductivity  as  that  of  a  copper  wire,  is  a  very  objectionable 
feature  in  that  it  gives  the  insulators  and  poles,  or  other  supports, 
a  far  greater  burden  than  is  necessary. 

The  resistance  of  a  conductor  varies,  of  course,  inversely  as  the 
conductivity,  and  therefore  inversely  as  the  cross-sectional  area 
of  a  uniform  wire.  Since  the  weight  also  varies  with  the  cross- 
section,  it  follows  that  the  resistance  of  a  wire  varies  inversely  as 
its  weight  per  mile.  A  very  convenient  method  of  comparing 
the  relative  resistance  of  various  grades  of  metals  used  in  making 
wire  is  to  take  as  the  standard  of  conductivity  the  weight  per 
mile-ohm.  The  weight  per  mile-ohm  of  a  conductor  is  the  weight 
of  a  conductor  a  mile  long,  and  of  such  uniform  cross-section  as 
to  have  a  resistance  of  one  ohm.  Evidently  the  better  the 
conductor,  the  smaller  such  a  wire  would  be,  and  therefore  a  low 
value  of  the  weight  per  mile-ohm  will  indicate  a  high  conductivity. 
The  relative  conductivities  of  any  two  metals  may  be  determined, 
knowing  the  weight  per  mile-ohm  of  each.  Thus,  if  the  weight 
per  mile-ohm  of  pure  copper  is  873.5  and  that  of  a  sample  wire 
is  896,  then  calling  the  conductivity  of  pure  copper  100  per 

*>_-,     - 

cent,  the  conductivity  of  the  sample  will  be          '     X   100  =  97 


per  cent. 

In  making  conductivity  tests,  the  resistance  of  the  sample  tested 
is  measured,  and  from  it  is  calculated  the  weight  per  mile-ohm 
for  that  sample.  This  value  can  then  be  compared  with  the 
weight  per  mile-ohm  of  pure  copper  as  in  the  above  example. 
By  doing  this  the  trouble  of  calculating  the  resistance  of  a  pure 
copper  wire  of  the  same  dimensions  as  that  of  the  sample  is 
saved. 

The  diameter  of  wire  for  electrical  purposes  is  usually  ex- 
pressed according  to  some  gauge,  and  there  are,  unfortunately,  a 
number  of  such.  Most  of  the  different  gauges  have  been  brought 
into  existence  by  various  wire  manfacturers  and  used  in  connec- 
tion with  their  particular  products  only.  In  these  guages  the 
sizes  of  wires  are  referred  to  by  numbers,  and  in  nearly  every 
case  the  smaller  numbers  refer  to  the  larger  wires.  A  better 
way,  and  one  which  is  coming  into  more  common  use,  is  to  refer 
to  the  diameter  in  thousandths  of  an  inch  or  in  mils,  as  thou- 
sandths of  an  inch  are  called.  A  very  convenient  way  of 
expressing  the  area  of  a  wire  is  to  give  its  cross-section  in  cir- 
cular mils  ;  a  circular  mil  being  the  area  of  a  circle,  the  diameter 
of  which  is  one  mil,  or  ^-^  of  an  inch.  This  is  better  than 
expressing  the  area  in  square  inches,  because  the  area  in  circular 


WIRE   FOR    TELEPHONE    USE.  349 

mils  is  obtained  simply  by  squaring  the  diameter  of  the  conductor 
in  mils.  This  very  simple  relation  between  the  area  in  circular 
mils  and  the  diameter  in  mils  is  true,  because  the  area  of  two 
circles  are  to  each  other  as  the  square  of  their  diameters.  To 
reduce  the  area  expressed  in  circular  mils  to  square  inches, 


7T 


multiply  it  by      or  .7854. 

4 

It  is  a  matter  of  importance,  when   purchasing  wire   in   any 
quantity,  to  measure  its  diameter  accurately,  so  as  to  be  sure  of 


Fig.  268. — Circular  Wire  Gauge. 

obtaining  the  size  ordered.  It  is  not  an  uncommon  thing  to 
order  a  wire  in  one  gauge  and  have  your  order  filled  in  another, 
and  the  latter  gauge  usually  happens  to  be  smaller  than  the 
former. 

Circular  wire  gauges,  such  as  is  shown  in  Fig.  268,  are 
obtainable,  and  serve  their  purpose  well,  but  are  subject  to  the 
disadvantage  that  a  separate  gauge  is  necessary  for  each  partic- 
ular set  of  gauge  numbers.  These  gauges  are  used  by  inserting 
the  wire  into  the  notches  in  its  periphery  until  one  is  found  which 
it  just  fits ;  the  number  corresponding  to  that  notch  is  then  the 
gauge  number  of  the  wire.  A  far  better  gauge,  although  one 
which  is  at  first  a  little  puzzling  to  use,  is  that  shown  in  Fig.  269 
and  known  as  the  micrometer.  It  consists  of  a  yoke  of  tempered 
steel,  in  one  side  of  which  is  mounted  agraduated  thumbscrew.  The 
wire  or  other  object  to  be  measured  is  placed  between  the  end  of 
the  thumbscrew  and  the  anvil  on  which  it  rests  when  closed,  and 
the  screw  turned  until  it  makes  light  contact  with  the  object  on 
both  sides.  These  screws  are  arranged  with  forty  threads  to  the 
inch,  so  that  one  complete  turn  of  the  screw  in  a  left-handed  direc- 
tion will  open  the  micrometer  -fa  of  an  inch.  The  edge  of  the  collar 
carried  by  the  screw  is  divided  into  twenty-five  equal  parts,  so 
that  a  turn  of  the  screw  through  one  of  these  divisions  will  open 
the  micrometer  -fa  of  -fa,  or  y^Vrr  of  an  inch.  The  shaft  on  which 


350  AMERICAN    TELEPHONE  PRACTICE, 

the  collar  turns  is  divided  into  tenths  of  an  inch,  and  each  T^ 
is  subdivided  into  four  parts.  Thus  a  rotation  of  twenty-five 
divisions  on  the  collar  will  equal  one  division  on  the  shaft,  or  .025 
inch.  If  the  collar  is  turned  so  as  to  expose  the  first  division  on 
the  shaft  and  thirteen  divisions  on  itself,  then  the  distance  which 
the  jaws  have  opened  will  be  equal  to  .025  -(-  .013  =  .038. 

The  Brown  &  Sharpe  gauge,  usually  abbreviated  B  &  S.,  is  prob- 
ably used  more  for  copper  wire  than  any  other  gauge,  while  the 
Birmingham  Wire  Gauge,  abbreviated  B.  W.  G.,  is  used  to  a 
greater  extent  for  iron  wire. 

A  decided  advantage  in  the  B.  &  S.  gauge  over  any  of  the 
others  is  that  the  areas  of  the  cross-sections  of  the  various  sizes 


Fig.  269. — Micrometer. 

of  wire  diminish  according  to  a  geometrical  progression  as  the 
gauge  number  increases.  The  ratio  in  this  progression  is  1.26, 
or  more  accurately  the  cube  root  of  two.  From  this  it  follows 
that  when  we  have  increased  three  sizes  we  have  doubled  the 
sectional  area  of  the  wire ;  and,  on  the  other  hand,  when  we  have 
diminished  three  sizes  we  have  reduced  the  cross-section  one- 
half.  A  very  convenient  thing  to  remember  in  the  B.  &  S. 
gauge  in  connection  with  copper  wire  is  that  the  diameter  of  a  No. 
10  wire  is  TV  of  an  inch  and  that  the  resistance  per  thousand  feet 
of  this  wire  is  one  ohm.  These  figures  are  not  perfectly  accurate, 
but  enough  so  for  most  practical  purposes.  If  one  desires  to  make 
an  approximate  calculation  regarding  the  size  of  any  wire,  he 
may  do  so  by  remembering  these  figures,  which  is  readily  done 
because  of  the  number  of  times  the  number  ten  occurs  in  them. 
For  example,  suppose  it  were  desired  to  find  the  resistance  of  a 
No.  13  B.  &  S.  gauge  copper  wire.  Inasmuch  as  13  is  three 
sizes  smaller  than  10,  the  area  of  a  No.  13  wire  will  be  one-half 
that  of  the  No.  10,  and  its  resistance  per  thousand  feet  double 
that  of  the  No.  10,  or  2  ohms.  If  the  resistance  of  a  No.  14 
instead  of  a  No.  13  were  desired,  it  could  be  found  by  finding  the 
resistance  of  a  No.  13  as  before  and  multiplying  by  1.26,  thus 
obtaining  the  result  2.52  ohms. 


WIRE   FOR    TELEPHONE    USE. 


35  [ 


Table   II.  gives  the  relative  sizes  of  various  numbers  of  wire 
in  several  of  the  gauges  which  are  or  have  been  in  use  in  this 

country. 

TABLE  II. 

TABLE  SHOWING   DIFFERENCE  BETWEEN  WIRE  GAUGES  IN  DECIMAL  PARTS  OF 

AN      NCH. 


D 

bo 

3 

c8 

O 

<u 

1* 

£ 

*0 

1 

American  or  Brown 
&  Sharpe. 

Birmingham  or 
Stubs'. 

Washburn  &  Moen 
Manufacturing 
Co.,  Worcester, 
Mass. 

Trenton  Iron  Co., 
Trer.ton,  N.  J. 

New  British, 
or  Standard. 

Old  English  from 
Brass  Mfrs. 
List. 

<u 

(H 

£ 

O 

6 
£ 

000000 



.46 

.... 

.464 

000000 

0000 
000 

oo 

.46 
.40964 
.3648 

•454 
•425 
•38 

•393 
•362 
•331 

•36 

•33 

•4 
•372 
•348 

.... 

.... 

0000 

ooo 

00 

O 
2 

3 
4 

•32495 
.2893 
•25763 
.22942 
.20431 

•34 
•3 
.284 
•259 
.238 

•307 
.283 
.263 
.244 
.225 

•305 
.285 
.265 
•245 
•  225 

•324 
•3 
.276 
.252 
•  232 

... 

O 

I 

2 

3 
4 

I 

7 
8 
9 

.18194 
.  16202 
.14428 
.12849 
•11445 

.22 
•203 
.18 
•165 
.148 

.207 
.192 
.177 
.162 
.148 

•  205 
.19 

•i?5 

.16 

•H5 

.212 
.IQ3 

.176 
.16 

.144 

.... 

5 
6 
7 
8 
9 

10 

ii 

12 

*3 

*4 

.10189 

.OQ0742 

.080808 

.071961 
.064084 

•I34 

.12 

.109 

•095 
.083 

•i35 

.  12 

.105 
.OQ2 
.08 

•13 
•"75 

.105 
•  0925 
.08 

.128 
.116 

.104 
.og2 
.08 

.'083 

10 
ii 

12 
13 
14 

15 
16 
i? 
18 
19 

.057068 
.05082 
•045257 
.040303 
.03589 

.072 
•065 
.058 
•°49 
.042 

.072 
.063 
•054 
.047 
.041 

•°7 
.061 

•0525 
•045 
•039 

.072 
.064 
.056 
.048 
.04 

.072 
•065 
.058 
•  049 
.04 

li 

17 
18 
19 

20 
21 
22 
2.3 
24 

.031961 

.028462 

.025347 
.022571 

.0201 

•035 
.032 
.028 
•  025 
.022 

•°35 
.032 
.028 
.025 
.023 

•°34 
•°3 
.027 
.024 
.0215 

•036 
.032 
.028 
.024 
.022 

•035 
•0315 
.0295 
.027 
.025 

20 
21 
22 
23 

24 

25 
26 

•  28? 
29 

.0179 
•01594 
.014195 
.012641 
.011257 

.02 
.Ol8 
.Ol6 
.014 
.013 

.02 
.Ol8 
.017 
.Ol6 
•015 

.019 
.018 
.017 
.016 
.015 

.02 
.018 
.0164 
.0148 
.0136 

.023 
.0205 
.01875 
.0165 
•OI55 

25 
26 
27 
28 
29 

3» 
3i 
32 
33 
34 

.OIOO25 
.008928 
.00795 
.00708 
.006304 

.OI2 
.OI 
.009 
.008 
.007 

.014 
•0135 
.013 
.Oil 
.01 

.014 
.013 
.012 

.Oil 
.01 

.0124 
.OIl6 
'  .0108 
.01 
.OO92 

•OI375 
.01225 
.01125 
.01025 
.0095 

3° 
3i 
32 
33 
34 

9 

11 

39 

.005614 
.005 
•004453 
.003965 
•003531 

.005 
.004 

.0095 
.009 
.0085 
.008 
.0075 

.009 
.008 
.00725 
.0065 
•00575 

.0084 
.0076 

.0068 
.006 

.00r,2 

.009 
.0075 
.0065 
•00575 
.005 

If 

37 
38 
39 

40 

.003144 

.007 

.005 

.0048 

•  0045 

40 

352  AMERICAN    TELEPHONE   PRACTICE. 

IRON   WIRE. 

Iron  wire  corrodes  so  rapidly  that  it  would  be  utterly  useless 
for  outdoor  work  were  it  not  possible  to  protect  it  to  some 
extent  from  the  action  of  the  weather.  This  is  done  by  a  proc- 
ess called  galvanizing,  which  consists  in  coating  wire  with  a  thin 
film  of  metallic  zinc.  The  process  of  manufacturing  iron  wire  is 
briefly  as  follows  :  the  iron,  after  being  brought  into  the  proper 
condition  by  various  processes  of  rolling  and  purifying,  is  rolled 
into  small  rods,  after  which  it  is  subjected  to  the  process  of 
"  drawing."  This  process  consists  in  pulling  the  rods  through  a 
series  of  dies,  made  of  steel,  each  die  being  smaller  than  the  one 
preceding  it.  This  is  necessarily  done  while  the  iron  is  cold  and 
is  termed  "  cold  drawing."  The  successive  drawings  of  the 
wire  through  the  dies  serves  not  only  to  reduce  its  cross-section, 
but  also  to  render  it  excessively  hard  and  brittle,  and  it  is  neces- 
sary, therefore,  to  anneal  it  frequently  between  the  drawings. 
After  the  wire  has  been  drawn  to  the  proper  size  it  is  annealed 
and  inspected  and  is  then  ready  for  galvanizing.  The  wire,  in 
order  to  thoroughly  clean  its  surface,  is  "  pickled  "  in  diluted 
sulphuric  acid  for  a  considerable  length  of  time,  after  which  it  is 
thoroughly  washed  in  order  to  remove  all  traces  of  acid.  It  is 
then  immersed  in  hydrochloric  acid.  The  wire  is  then  rolled 
from  one  reel  to  another  and  between  these  reels  it  passes  first 
through  a  furnace  heated  to  a  very  high  degree,  immediately 
afterward  through  a  vat  containing  a  solution  of  hydrochloric 
acid  which  cools  the  wire  and  removes  any  oxides  that  have 
formed  during  the  drawing,  and  then  through  a  second  vat  con- 
taining molten  zinc  maintained  at  a  constant  temperature  by  a 
furnace  underneath.  The  time  between  the  immersion  in  the 
last  acid  bath  and  the  zinc  acid  bath  is  short,  because  these 
vats  are  placed  very  close  together,  and  the  metal  therefore  has 
no  chance  to  oxidize. 

As  the  proper  galvanizing  of  iron  wire  is,  all  things  considered, 
the  most  important  step  in  its  manufacture,  it  is  very  essentfel 
that  reliable  tests  are  made  before  purchasing  wire  for  outdoor 
use.  Fortunately  such  a  test  is  a  very  easy  thing  to  make,  but, 
unfortunately  for  the  ordinary  purchaser,  they  are  very  seldom 
made.  Several  samples  of  the  wire  should  be  selected  at 
random.  Each  should  then  be  immersed  in  a  strong  solution  of 
sulphate  of  copper  for  a  period  of  seventy  seconds.  It  should 
then  be  withdrawn  and  wiped  clean  with  a  cloth.  This  process 
is  repeated  in  all  four  times.  If,  at  the  end  of  the  fourth  immer- 


WIRE   FOR    TELEPHONE    USE.  353 

sion,  the  wire  appears  black,  as  it  did  at  the  end  of  the  first 
immersion,  the  zinc  has  not  all  been  removed  and  the  galvanizing 
may  be  said  to  have  been  well  done ;  but  if  the  wire  has  a  copper 
color,  either  as  a  whole  or  in  spots,  it  shows  that  the  zinc  has 
been  eaten  away  and  that  copper  has  deposited  itself  upon  the 
iron  wire.  In  this  case  the  wire  should  be  rejected. 

Iron  wire  which  is  thoroughly  well  galvanized  is  at  best  rather 
short-lived,  and  poor  galvanization  may  result  in  the  total  loss  of 
the  wire  within  a  year.  Well  galvanized  iron  wire  has  been 
known  to  last  twelve  years,  but  the  conditions  were  very  favor- 
able. Four  to  six  years  probably  represents  a  fair  average  for 
the  life  of  wire  of  this  kind,  but  cases  are  frequent  where  wires 
have  been  so  corroded  within  a  year  as  to  make  their  replacement 
necessary.  In  factory  districts  and  in  railroad  yards  where  the 
gases  from  furnaces  come  in  constant  contact  with  the  wire,  the 
life  of  the  zinc  coatingtis  very  short. 

The  grades  of  galvanized  iron  wire  as  used  by  the  manu- 
facturers are,  if  not  well  understood,  very  misleading.  'They  are 
referred  to  in  the  following  terms  :  Extra  Best  Best,  Best  Best, 
Best,  and  Steel,  the  first  three  in  this  list  being  abbreviated  E. 
B.  B.,  B.  B.,  and  B. 

Extra  Best  Best  wire  is  of  a  very  soft,  high  grade  material, 
having  the  highest  conductivity  of  all.  It  has  sufficient  tensile 
strength  for  all  ordinary  purposes,  while  its  conductivity  is  far 
superior  to  that  of  the  other  grades.  It  has  a  breaking  strength 
of  three  times  its  weight  per  mile,  and  the  weight  per  mile-ohm 
varies  from  4500  to  4800,  4700  being  a  good  average. 

Best  Best  is  less  uniform  and  tough  than  the  above,  but  is 
somewhat  better  mechanically.  It  has  a  breaking  strength  of 
about  3.3  times  its  weight  per  mile,  and  its  weight  per  mile-ohm 
varies  from  5300  to  6000. 

Best  should  undoubtedly  have  been  called  worst,  for  as  a  rule 
it  is  a  rather  poor  quality  of  wire,  and  before  accepting  it  it 
should  be  very  carefully  tested.  It  is  harder  and  less  pliable 
than  the  preceding  grades,  and  has  a  weight  per  mile-ohm  of 
about  6500. 

Steel  wire,  which  is  in  reality  a  rather  low  grade  Bessemer  proc- 
ess wire,  is  much  stronger  than  any  of  the  above  grades,  but  is 
greatly  lacking  in  conductivity.  It  has  a  breaking  strength  of 
about  five  and  one-half  times  its  weight  per  mile,  and  its  weight 
per  mile-ohm  varies  between  6000  and  7000  pounds. 

Steel  wire  is  largely  used  for  telephone  work  on  very  short 
lines,  and  if  well  galvanized  serves  its  purpose  admirably.  In 


354  AMERICAN   TELEPHONE  PRACTICE. 

short  city  lines  no  difference  can  be  noticed  so  far  as  talking 
results  are  concerned  between  an  iron  or  steel  and  a  copper  cir- 
cuit. The  steel  wire  is,  as  a  rule,  cheaper  than  an  Extra  Best  Best 
or  the  Best  Best,  and  has  the  additional  advantage  of  greater 
mechanical  strength. 

The  following  specifications  are  in  substance  those  used  by 
the  Western  Union  Telegraph  Company  in  selecting  their  iron 
wire  : 

(1)  The  wire  shall  be  soft  and  pliable,  and  capable  of  elongat- 
ing fifteen  per  cent,  without  breaking,  after  being  galvanized. 

(2)  Great  tensile  strength  is  not  required,  but  the  wire  must 
not  break  under  a  less  strain  than  two  and  one-half  times    its 
weight  in  pounds  per  mile. 

(3)  Tests  for  ductility  will  be  made  as  follows  :   Pieces  of   wire 
shall  be  gripped  by  two  vises  six  inches  apart  and  twisted.     The 
full  number  of  twists  must  be  distinctly  visible  between  the  vises 
on    the  six-inch    piece.     The    number  of   twists    in  a  piece    six 
inches  long  shall  not  be  under  fifteen. 

(4)  The  electrical  resistance  of  the  wire  in  ohms  per  mile  at  a 
temperature   of   68    degrees    Fahrenheit    must    not    exceed    the 
quotient  arising  from  dividing  the  number  4800  by  the  weight 
of  the  wire  in  pounds  per  mile.     This  is  equivalent  to  saying 
that  the  weight  per  mile-ohm  must  not  exceed  4800.     The  co- 
efficient   .003    will    be    allowed    for   each  degree    Fahrenheit  in 
reducing  to  a  standard  temperature. 

(5)  The  wire  must  be  well  galvanized  and  capable  of  standing 
the  test  of  dipping  into  sulphate  of  copper  as  stated  above. 

The  British  Post  Office  Specifications  require  a  value  of  the 
weight  per  mile-ohm  of  5323. 

Table  III.,  taken  from  Roebling,  gives  the  weight,  breaking- 
strength,  and  resistance  of  the  various  sizes  and  grades  of  galvan- 
ized iron  wire  : 


WIRE   2?OR    TELEPHONE    USE. 


355 


TABLE  III. 
GALVANIZED  IRON  WIRE. 


d 

§ 
p 

Weights, 
Pounds. 

Breaking 
Strengths, 
Pounds. 

Resistance  Per  Mile 
in  Ohms. 

PQ 

& 

| 

!_ 
0) 

1 

1 

s 

1000 

Feet. 

One 
Mile. 

Iron. 

Steel. 

IE.  B.  B. 

B.  B. 

Steel. 

1 

0 

340 

304 

1607 

4821 

9079 

2-93 

3-42 

4-05 

i     |  300 

237 

1251 

3753 

7068 

3-76 

4.4 

5-2 

2          284 

212 

II2I 

3363 

6335 

4.19 

4.91 

5-8 

3        259 

177 

932 

2796 

5268            5.04 

5-9 

6-97 

4        238 

149 

787 

2361 

4449 

5-97 

6-99 

8.26 

5 

220 

127 

673 

2019 

3801 

6.99 

8.18 

9.66 

6 

2O3 

109 

573 

1719 

3237 

8.21 

9.6 

"•35 

7 

1  80 

85 

450 

1350 

2545 

10.44 

12.21 

14-43 

8 

165 

72 

378 

H34 

2138 

12.42 

14-53 

17.18 

9 

148 

58 

305 

915 

1720 

15-44 

18.06 

21-35 

10 

134 

47 

250 

750 

1410 

18.83 

22.04 

26.04 

ii 

120 

38 

200 

600 

1131 

23-48 

27.48 

32.47 

12 

lOg 

31 

165 

495 

933 

28.46 

33-3 

39.36 

13 

95 

24 

125 

375 

709 

37-47 

43.85 

51.82 

14 

83 

18 

96 

288 

54i 

49.08 

57-44  1 

67.88 

15 

72 

13-7 

72 

216 

407 

65.23 

76.33 

90.21 

16 

65 

ii  .1 

59 

177 

332 

80.03 

93-66 

110.7 

17 

58 

8.9 

47 

141 

264 

100.5 

120.4 

139- 

18 

49 

6-3 

33 

99 

189 

140.8 

164.8 

193.8 

COPPER   WIRE. 

Copper  wire  is  practically  indestructible  by  exposure  to  ordi- 
nary climatic  influences.  After  it  is  first  put  up  it  acquires  a  thin 
coating  of  oxide,  and  after  that  no  change  whatever  takes  place, 
so  far  as  can  be  ascertained.  The  process  of  maTiufacturing 
copper  wire  is  similar  to  that  for  iron  wire,  with  the  exception 
that  no  galvanizing  is  necessary.  The  process  of  drawing  copper 
wire  has  been  so  greatly  improved  recently  that  the  old  fault, 
lack  of  mechanical  strength,  has  been  almost,  if  not  quite,  over- 
come. Copper  wire  is  now  drawn  so  as  to  possess  a  breaking 
strength  of  60,000  pounds  per  square  inch,  which  is  quite  equal 
to  that  of  some  grades  of  iron  wire.  The  difference  between 
hard-drawn  copper  wire  and  soft  wire  is  due  entirely  to  the  fact 


356 


AMERICAN   TELEPHONE  PRACTICE. 


that  the  hard-drawn  wire  is  not  annealed  as  often  between  the 
drawings.  The  value  of  the  weight  per  mile-ohm  is,  for  good 
commercial  wire,  682  pounds,  the  wire  having  a  tensile  strength 
equal  to  about  three  times  its  weight  per  mile.  For  hard-drawn 
wire  the  percentage  of  elongation  is  not  nearly  so  high  as  that 
for  iron  wire,  being  only  about  one  per  cent,  before  breaking. 

The  value  in  pounds  per  mile-ohm  of  pure  annealed  copper  is 
859,  this  being  based  on  the  international  ohm. 

In  the  following  table,  taken  from  Roebling's  "  Wire  in  Elec- 
trical Construction,"  the  weights  and  resistances  of  the  various 
B.  &  S.  gauge  numbers  of  copper  wire  are  given  : 

TABLE  IV. 
COPPER  WIRE  TABLE. 


C/2 

i 

5 

Weights 
per 

Resistances  per  1000  Feet 
in  International  Ohms 

K  o> 

g 

o 
•**  t/5 

as  *> 

in 

IH 

O-2 

0  OS 

x>o 

.ss 

g 

1 

Cj 

5 

.5 

Q) 

IH 

looo  feet. 

Mile. 

At  60°  F. 

At  75°  F. 

5 

0000 

46o. 

2II600. 

641. 

3382. 

.04811 

.04966 

000 

410. 

l68lOO. 

509- 

2687. 

.06056 

.06251 

oo 

0 

365. 
325- 

133225. 
105625. 

403- 

320. 

2129. 
1688. 

.07642 
.09639 

.07887 
.09948 

I 

289. 

83521. 

253- 

1335- 

.1219 

.1258 

2 

258. 

66564. 

202. 

1064. 

.1529 

•I579 

3 

229. 

5244I- 

I59. 

838. 

.1941 

.2004  ; 

4 

204. 

4l6l6. 

126. 

665. 

.2446 

•2525 

5 

182. 

33124. 

100. 

529- 

•3°74 

•  3172 

6 

162. 

26244. 

79- 

419. 

•3879 

.4004 

7 
8 
9 

144. 
128. 
114. 

20736. 
16384. 
12996. 

63. 
SO- 

39- 

262! 
208. 

.491 
.6214 

•7834 

.5067 
.•6413 
.8085 

10 

IO2. 

10404. 

32- 

166. 

•9785 

.01 

ii 

QI. 

828l. 

25- 

132. 

1.229 

.269 

12 

81. 

6561. 

20. 

105. 

1-552 

.601 

13 

72. 

5184- 

15-7 

83- 

1.964 

.027 

14 

64- 

4096. 

12.4 

65- 

2.485 

.565 

J5 

57- 

3249- 

9.8 

52. 

3-133 

3.234 

16 

5i- 

26OI  . 

7-9 

42. 

3  .914 

4.04 

17 

45- 

2025. 

6.1 

32. 

5.028 

5.189 

18 

40. 

I6OO. 

4.8 

25.6 

6.363 

6.567 

19 

36. 

1296. 

3-9 

20.7 

7.855 

8.108 

20 

32. 

1024. 

3-1 

16.4 

9.942 

10.26 

21 

38.5 

8l2.3 

2-5 

13- 

12.53 

12.94 

22 

25-3 

640.1 

i.  9 

10.2 

15-9 

16.41 

23 

22.6 

510.8 

i  .5 

8.2 

19.93 

20.57 

24 

20.1 

404. 

1.2 

6-5 

25-2 

26.01 

a 

27.9 
15-9 

320.4 
252.8 

•97 
•77 

4- 

31.77 
40.27 

32.79 
41.56 

27 

I4.2 

201.6 

.61 

3-2 

50.49 

52.11 

28 

12.6 

158.8 

.48 

•5 

64.13 

66.18 

29 

"•3 

127.7 

•39 

79-73 

82.29 

3° 

10. 

100. 

•  3 

'.6 

101  .8 

105.1 

31 

18.9 

79.2 

.24 

.27 

128.5 

132.7 

32 

8. 

64. 

i         *19 

.02 

164.2 

33 

7.1 

50-4 

.81 

202. 

208.4 

34 

6-3 

39-7 

.12 

•63 

256-5 

264.7 

y 

5-6 
5- 

3i-4 
25- 

•095 
.076 

•  5 

•4 

324.6 
407.2 

335-i 
420.3 

WIRE   FOR    TELEPHONE    USE.  357 

Abbott  gives  the  following  specifications  governing  the  require- 
ments to  be  made  of  manufacturers  in  purchasing  copper  wire  : 


COPPER   WIRE. 

i 

1.  Finish. — Each    coil  shall  l?e  drawn  in  one  length  and  be 
exempt   from  joints  or  splices.      All  wire  shall  be    truly  cylin- 
drical and  fully  up  to  gauge  specified  for  each  size,  and  must  not 
contain  any  scale,  inequalities,  flaws,  cold  shuts,  seams,  or  other 
imperfections. 

2.  Inspection. — The  purchaser  will  appoint  an  inspector,  who 
shall  be  supplied  by  the  manufacturer  with  all  facilities  which 
may  be  required  for  examining  the  finished  product  or  any  of 
the    processes  of   manufacture.     The    inspector   shall    have  the 
privilege  of  overseeing  the  packing  and  shipping  of  the  samples. 
The  inspector  will  reject  any  and  all  wire  which  does  not  fully 
come  up  to  all  the  specification  requirements.     The  purchaser 
further  reserves  the  right  to  reject  on  reception   any  or  all  lots 
of  wire  which  do  not  fulfill  the  specifications,  even  though  they 
shall  previously  have  been  passed  or  accepted  by  the  inspector. 

3.  Apparatus. — The  manufacturer  must  supply,  at  the  mill,  the 
necessary  apparatus  for  making  the  examination  called  for.     This 
apparatus  shall  consist  of  a  tension-testing  machine,  a  torsion- 
testing  machine,  an  elongation  gauge,  an  accurate  platform  scale, 
and   an  accurate  bridge  and  battery.     Each  of  these  pieces  of 
apparatus  may  be  examined  by,  and  shall  be  satisfactory,  to  the 
inspector. 

4.  Packing  for  Shipment. — When  ready  for  shipment  each  coil 
must  be  securely  tied  with  not  less  than  four  separate  pieces  of 
strong  twine  and  shall  be  protected  by  a  sufficient  wrapping  of 
burlap  so   the  wire  may   not  be  injured   during  transportation. 
The  wrappings  shall  be  placed  upon  the  wire  bundles,  after  they 
have  been  coiled  and  secured  by  the  twine.     The  diameter  of 
the  eye  of  each  coil  shall  be  prescribed  by  the  inspector,  and  all 
coils  shipped  shall  not  vary  more  than  two  inches  in  the  diame- 
ter of  the  eye. 

5.  Weight. — Each  coil  shall  have  its  length  and  weight  plainly 
and  indelibly  marked  upon  two  brass  tags,  which  shall  be  secured 
to  the  coil,  one  inside  the  wrapping  and  the  other  outside. 

6.  Mechanical  Properties. — All  wire  shall  be  fully  and  truly  up 
to   guage   standard,  as  per  B.  &  S.  wire  guage.     The  wire  shall 
be  cylindrical  in  every  respect.     The  inspector  shall  test  the  size 
and  roundness  of  the  wire  by  measuring  both  ends  of  each  coil, 


358 


AMERICAN    TELEPHONE  PRACTICE. 


and  also  by  measuring  at  least  four  places  in  the  length  of  each 
coil.  A  variation  of  not  more  than  \\  mil  on  either  side  of  the 
specified  wire-guage  number  will  be  allowed,  and  the  wire  must 
be  truly  round  within  one  mil  upon  opposite  diameters  at  the 
same  point  of  measurement.  The  strength  of  the  wire  shall  be 
determined  by  taking  a  sample  from  one  end  of  each  coil,  30"  in 
length.  Of  this  piece,  18"  shall  be  tested  for  tension  and  elonga- 
tion, by  breaking  the  same  in  the  tension-testing  machine.  The 
samples  should  show  a  strength  in  accordance  with  the  following 
table  : 

TABLE  V. 
BREAKING  WEIGHT  OF  HARD-DRAWN  AND  ANNEALED  COPPER  WIRE. 


Size  of  Wire,  B.  &  S. 
Guage. 

Breaking       Weight       of 
Hard-Drawn  —  Pounds. 

Breaking        Weight      of 
Annealed  —  Pounds. 

oooo 

997i 

5650 

000 

7907 

4480 

00 

6271 

3553 

o 

4973 

2818 

i 

3943 

2234 

2 

3>27 

1772 

3 

2480 

1405 

4 

1967 

1114 

5 

1559 

883 

6 

'237 

700 

7 

080 

555 

8 

778 

440 

9 

017 

349 

10 

489 

277 

ii 

388 

219 

12 

307 

174 

13 

244 

138 

14 

193 

109 

15 

153 

87 

16 

133 

69 

J7 

97 

55 

18 

77 

43 

19 

61 

34 

20 

48 

27 

A  variation  of  i£  per  cent,  on  either  side  of  the  tabular  limits 
will  be  accepted  by  the  inspector.  The  elongation  of  the  wire 
must  be  at  least  three  per  cent,  for  all  sizes  larger  than  No.  i  ; 
ij  per  cent,  from  No.  i  to  No.  10,  and  i  per  cent,  for  sizes  less 
than  No.  10,  for  hard-drawn  copper  wire.  The  remainder  of  the 
sample  selected  will  be  tested  for  torsion.  The  torsion  sample 
will  be  twisted  in  the  torsion-testing  machine,  to  destruction,  one 
foot  in  length  being  placed  between  the  jaws  of  the  machine. 
Under  these  circumstances  hard-drawn  copper  wire  shall  show 


WIRE   FOR    TELEPHONE    USE. 


359 


not  less  than  20  twists  for  sizes  over  No.  I  ;  from  40  to  90  twists 
in  sizes  from  No.  I  to  No  10;  and  not  less  than  100  twists  in 
sizes  less  than  No.  10.  Should  the  sample  selected  from  one 
end  of  each  coil  show  failure  to  come  up  to  the  specifications, 
the  inspector  may  take  a  second  sample  from  the  other  end  of 
the  coil.  If  the  average  of  the  results  from  both  samples  shall 
be  within  the  specifications,  the  coil  shall  be  accepted  ;  if  not 
within  the  specifications,  the  coil  shall  be  rejected.  The  weight 
per  mile  shall  be  determined  by  carefully  weighing  2  per  cent,  of 
the  number  of  coils  called  for  in  the  contract,  and  the  weight 
thus  obtained  shall  correspond,  within  2  per  cent,  on  either  side. 
of  the  result  given  in  the  following  formulae  : 


\\r   -    u^  -i 

Weight  per  mile  =  =  ~ 


CM 
Weight  per  1000  ft.  = 


330-353 

7.  Electrical  Properties. — The  electrical  properties  of  the  wire 
shall  be  determined  by  the  inspector  selecting  3  per  cent,  of  the 
coils,  and  from  them  taking  lengths  of  100  ft.,  500  ft.,  or  1000 
ft.,  at  his  discretion,  and  measuring  the  conductivity  of  the  same 
with  a  standard  bridge.  For  soft-drawn  copper  wire  the  follow- 
ing resistance  per  mil-foot  will  be  assumed  : 

TABLE  VI. 
RESISTANCE  OF  COPPER  WIRE  AT  VARIOUS  TEMPERATURES. 


a 

a 

e 

"3 

£fe 

s 

'S&i 

3 

5  tfl 

*  ?/i 

53    ^ 

-  «5 

rt   0) 

I 

i)    r^ 

si 

50 

2 
'53 

ss 

|! 

II 
50 

</3 

H 

o 

H 

0) 

0 

8.96707 

60 

10.20253 

10 

9.16413 

70 

10.42083 

20 

9-36473 

80 

10.64268 

30 

9-56887 

90 

10.86806 

40 

9.77655 

IOO 

11.09698 

50 

9.98777 

For  hard-drawn  wire  the  resistance  per  mil-foot  shall  be  1.0226 
times  the  foregoing  figures.  All  wire  shall  be  within  98  per 
cent,  of  the  above  figures. 


CHAPTER   XXX. 

POLE-LINE   CONSTRUCTION. 

THE  poles  most  used  in  the  United  States  are  of  Norway  pine, 
chestnut,  cedar,  and  cypress.  Southern  pine  is  not  as  durable 
as  Northern  pine,  although  it  is  used  to  a  large  extent  in  the 
South.  Canadian  cedar,  is,  however,  all  things  considered,  the 
best  wood  to  use. 

The  average  life  of  the  various  woods  mentioned  are,  accord- 
ing to  Maver,  as  follows  : 

Norway  Pine, 6  years 

Chestnut, 15 

Cedar, 12      " 

Cypress,  .         .         .         .         .         .         .         .  10      " 

In  choosing  the  kind  of  pole  to  be  used,  the  locality  must 
always  be  considered,  for  obviously  it  would  be  poor  economy 
to  bring  cedar  poles  from  Canada  for  the  reason  that  they  would 
!last  perhaps  a  few  more  years  than  cypress  poles,  which  could  be 
^cdt  on  the  ground. 

Poles  should  be  well  seasoned  before  setting  in  the  ground. 
This  is  either  accomplished  by  natural  process  of  drying,  or 
sometimes  in  a  special  drying  kiln.  Before  seasoning,  however, 
the  pole  should  be  peeled  and  all  knots  trimmed.  It  is  easier  to 
do  this  while  the  sap  is  in  them  than  afterwards,  and,  moreover, 
the  drying  takes  place  in  a  shorter  time  if  the  bark  is  removed. 
If  the  pole  is  not  seasoned  before  setting  or  before  it  is  painted, 
•where  it  is  to  be  painted,  the  sap  is  sure  to  cause  a  dry  rot, 
which  will  eventually  destroy  the  pole.  The  worst  feature 
•of  this  trouble  is  that  the  defect  is  not  noticeable  on  the 
surface  and  therefore  is  likely  to  cause  trouble  when  least 
expected.  A  pole  may  have  all  appearances  of  being  per- 
fectly sound  and  yet  be  a  mere  shell,  so  that,  when  subjected 
to  some  heavy  storm,  it  comes  down  on  the  line,  perhaps  bring- 
ing many  other  poles  with  it. 

Practice  differs  to  some  extent  concerning  the  size  of  poles. 
Money  saved,  however,  in  the  purchase  of  light  poles,  is  usually 


POLE-LINE    CONSTRUCTION. 


saved  at  a  great  cost  in  the  future.     Table  VII.  gives  a  list  of  the 
sizes  which  meet  the  demands  of  the  best  practice  to-day. 


TABLE  VII. 


Length. 

( 
Diam.  at  Top. 

Diam.  6  ft.  from  Butt. 

25  feet 

7  inches 

9  inches 

30 

7 

10 

35 

7 

ii 

40 

7 

12 

45 

7 

13 

50 

7 

14 

55 

7 

16 

60 

7 

17 

65 

7 

18 

70 

7 

20 

Telephone  companies  that  have  been  in  the  field  long 
enough  have  learned  that  the  days  of  "  fence-post  "  construc- 
tion are  over,  and  that  in  the  long  run  poor  construction  is  much 
more  expensive  than  good.  To  be  sure,  in  many  of  the  indepen- 
dent installations  it  is  a  matter  of  necessity  to  use  a -medium  con- 
struction throughout  on  account  of  the  first  expense,  and  in  such 
case  if  the  dimensions  of  the  poles  given  in  the  table  above  are 
too  expensive,  they  will  at  least  serve  as  a  standard  at  which  to 
aim.  In  many  cases  poles  with  5-inch  tops  will  meet  all  the  de- 
mands of  an  exchange,  for  a  few  years  at  least,  and  it  is  some- 
times expedient  to  use  them. 

The  number  of  wires  to  be  carried  on  any  pole  line  is  also 
a  question  that  will  largely  determine  the  diameter  of  the  poles. 
On  the  corners,  or  where  a  heavy  lead  is  dead-ended,  to  make 
connection  perhaps  with  an  underground  cable,  the  poles  used 
should  be  in  many  cases  much  larger  than  those  given.  In  fact, 
in  such  cases  the  heaviest  poles  that  can  be  had  will  be  none 
too  large,  and  it  is  not  uncommon  to  see  a  4O-foot  pole  with  an 
1 8-inch  top  at  critical  points  on  some  of  the  best  constructed 
heavy  lines. 

The  question  of  the  number  of  poles  to  the  mile  is  one  that 
must  be  decided  to  meet  the  particular  conditions  of  the  line 
to  be  erected.  The  greater  the  number  of  poles  the  lower  the 
insulation,  but  this  is  a  very  small  disadvantage,  and  is  more 
than  offset  by  the  greater  freedom  from  breakage  of  wires  and 
consequent  decrease  in  the  expense  of  maintenance  when  the 
poles  are  set  closely  together.  In  Europe  the  common  practice 


362 


AMERICAN   TELEPHONE   PRACTICE. 


is  to  use  as  few  as  twenty  poles  to  the  mile.  In  this  country, 
however,  the  best  practice  dictates  the  use  of  from  forty  to  fifty 
to  the  mile,  although  many  lines  are  successfully  operated  with 
thirty,  or  less.  As  a  rule,  the  greater  the  number  of  wires  carried, 
the  closer  and  heavier  the  poles  should  be.  The  liability  of  any 
particular  locality  to  heavy  sleet  and  wind  storms  is  another 
factor  in  determining  the  size  and  distribution  of  poles.  In  the 
long-distance  lines  of  the  American  Telegraph  and  Telephone 
Company  the  standard  distance  between  the  poles  is  130  feet, 
making  approximately  forty  to  the  mile.  The  standard  pole  is 


Fig.  270. — Pole  Equipped  with  Guards. 

35  feet  in  length,  and,  while  none  are  shorter  than  this,  many  are 
much  longer.  Seventy-foot  poles  are  often  used,  and  in  some 
cases  the  height  of  100  feet  is  reached. 

In  cities  poles  varying  from  40  to  60  feet  are,  as  a  rule,  used. 
These  are  generally  of  Norway  pine,  as  it  is  somewhat  difficult 
to  get  cedar  poles  of  this  height.  It  is  usually  necessary  to  use 
a  longer  pole  in  city  work,  in  order  that  the  line  may  be  carried 
above  the  city  electric  light  and  power  circuits,  and  also  that  the 
work  of  firemen  may  not  be  interfered  with.  It  is  well  to  pro- 
tect poles  along  the  streets  of  cities  from  the  gnawing  of 
horses  hitched  to  them,  and  also  from  the  wearing  effects  of 


POLE-LINE   CONSTRUCTION. 


363 


wagon-hubs,  which  often  very  greatly  weaken  the  poles  at  a 
point  where  they  are  least  able  to  stand  it.  Galvanized  steel 
protecting  strips  are  obtainable  for  the  former  purpose,  and 
what  are  termed  butt-plates,  about  15  inches  by  18  inches  by 
fV  inch  thick,  of  the  same  material,  may  also  be  purchased  from 
supply  dealers  for  the  latter  purpose.  A  pole  thus  equipped  is 
shown  in  Fig.  270. 

In  Table  VIII.  is  given  some  useful  information  concerning 
the  weights  of  poles  of  various  sizes  and  the  number  forming 
a  carload. 

TABLE  VIII. 
WOOD  POLES. — CEDAR. 


Length. 

Top. 

Weight  in  Ibs. 

No.  to  Carload. 

25  feet. 

5  inches. 

200 

1  20 

25 

6 

275 

no 

30 

6 

325 

100 

30 

7 

450 

So 

35 

6 

500 

120 

35 

7 

600 

no 

40 

6 

700 

IOO 

40 

7 

800 

90 

45 

6 

950 

82 

45 

7 

IIOO 

60 

50 

6 

1250 

40 

50 

7 

1450 

25 

55 

6 

1500 

30 

55 

7 

1  800 

25 

NORWAY  PINE. 


Length. 

Top. 

Weight  in  Ibs. 

No.  to  Carload. 

40  feet. 

7  inches. 

IIOO 

QO 

45 

7 

1  200 

80 

50 

7 

1350 

72 

55 

7 

1500 

65 

60 

7 

1700 

55 

65 

7 

2000 

45 

70 

7 

2400 

50 

75 

7 

2800 

45 

80 

7 

3400 

35 

85 

7 

t 

3800 

30 

It  is  not  customary,  in  this  country,  to  treat  poles  with  any 
preserving  process,  but  it  is  always  well  to  coat  the  pole  for  a 
distance  of  six  feet  from  the  butt  with  pitch,  before  setting  it. 


364 


AMERICAN    TELEPHONE  PRACTICE. 


It  is  also  well  to  give  city  poles  two  coats  of  good  oil  paint,  and 
a  very  neat  appearance  is  added  if  the  lower  portions  are  painted 
black  to  a  distance  of  six  feet  above  the  ground,  while  the 
remaining  portion  is  painted  some  light  color.  In  Europe  a 
process  termed  creosoting  is  meeting  with  great  favor  for  pre- 
serving telephone  and  telegraph  poles.  It  is  the  cheapest  of  all 
known  expedients  of  this  kind  and  consists,  briefly,  in  placing 
the  pole  in  an  iron  chamber  from  which  the  air  may  be  exhausted. 
This  causes  the  sap  and  all  other  juices  from  the  wood  to  ooze 
out  from  its  pores.  After  this  steam,  at  a  pressure  of  about 
100  pounds  to  the  square  inch,  is  admitted  to  the  cyclinder  and 
the  poles  are  subjected  to  this  treatment  for  about  four  hours. 
After  this  crude  petroleum  is  forced  into  the  cyclinder  under 
a  pressure  of  about  300  pounds  to  the  square  inch,  and  it  is 
found  that  it  penetrates  to  the  very  heart  of  the  poles,  thus 
adding  very  materially  to  their  lasting  qualities.  Cases  are  cited 
where  poles  treated  by  this  method  have  been  perfectly  sound 
after  having  been  in  service  for  a  period  of  twenty  years. 

Another  process,  termed  vulcanizing,  consists  in  heating  the 
pole  in  a  closed  vessel  for  several  hours  to  a  temperature  of 
about  500°  F.  The  principle  in  this  treatment  is  that  the  intense 
heat  causes  the  sap  in  the  wood  to  coagulate,  after  which  it  can 
produce  no  evil  effects.  This  would  apparently  be  cheaper  even 
than  the  creosoting. 

The  cross-arms  carrying  the  insulators  are  preferably  of  sawed 
yellow  pine.  Two  sizes  are  in  general  use,  the  standard  being 
4i  by  3J.  The  lengths  vary  from  3  to  10  feet,  according  to  the 
number  of  pins  or  insulators  to  be  used.  Table  IX.  shows  the 
lengths  of  the  various  standard  cross-arms ;  also  the  spacings  of 

the  pin-holes. 

TABLE  IX. 


Spacings. 

Length. 

Number  of 
Pins. 

End. 

Center. 

Sides. 

3  feet. 

2 

4  in. 

28    in. 

4 

4 

16 

12    in. 

5 

4 

18 

17 

6 

4 

22 

21 

6 

6 

16 

12 

8 

6 

18 

J7s 

8" 

8 

16 

12 

10 

8 

17* 

I5f 

10 

10 

16 

12 

POLE- LINE    CONS TR  UC  TION. 


365 


The  standard  size  of  pin  for  the  above  arm  has  a  i^-inch  shank, 
and  arms  of  this  size  are  usually  bored  accordingly.     They  are 


Fig.  271.— Four-Pin  Cross  Arm. 

also  bored  as  shown  in   Fig.  271  with  two  £-inch  holes  for  lag- 
screws  used  in  attaching  them  to  the  poles. 

Another  size  of  cross-arm,  called  the  telephone  arm,  has  come 
into  use  to  a  considerable  extent  for  cheaper  installation.  The 
size  of  this  arm  is  2|  by  3},  being  \  inch  smaller  in  each  dimen- 


Fig.  272.— Lag-Screw. 

sion  than  the  standard.  These  arms  are  usually  bored  for  i^- 
inch  pins  and  the  length  of  a  ten-pin  arm  is  only  &J  feet.  The 
various  dimensions  are  shown  in  Table  X. 

TABLE  X. 


Spacings. 

Length. 

Number  of 
Pins. 

i 

End. 

Center. 

Sides. 

24  in. 

2 

3  in- 

18  in. 

30 

2 

24 

36 

2 

30 

42 

4 

16 

10  in. 

62 

6 

16 

10    " 

82 

8 

16 

IO<  " 

102 

10 

16 

10    " 

120 

12 

16 

10    " 

All  cross-arms  should  be  given  two  coats  of  good  metallic 
paint,  usually  red,  before  setting  in  position.  In  order  to  attach 
them  to  the  pole  a  gain  is  cut  in  the  pole  of  such  dimensions  as 
to  accurately  fit  the  longest  side  of  the  cross-arm.  The  gain 
should  not  be  more  than  one  inch  deep,  however,  for  the  reason 
that  a  greater  depth  is  likely  to  weaken  the  pole  unduly.  The 
gain  should  be  given  two  coats  of  good  white  lead  before 
the  cross-arm  is  put  in  place.  The  common  way  of  attaching  the 


366  AMERICAN   TELEPHONE  PRACTICE, 

cross-arms  to  the  pole  is  by  two  lag-screws  of  the  type  shown  in 
Fig.  272.  These  are  of  such  length  as  to  reach  almost  through 
the  pole,  and  their  threads  are  cut  in  such  a  manner  that  they 
may  be  driven  part  of  the  way  home.  A  better  practice  now  is 
to  attach  the  cross-arm  to  the  pole  by  means  of  a  single  carriage 
bolt  extending  entirely  through  the  arm  and  pole,  being  secured 
by  a  nut  and  a  washer.  This  method  has  an  advantage  over 
the  use  of  lag-screws  in  that  the  hole  for  the  carriage  bolt  may 
be  bored  perfectly  smooth  and  clean,  and  of  such  size  as  to 
accurately  fit  the  carriage  bolt,  so  there  is  little  chance  for  rotting. 
A  slightly  better  way,  perhaps,  but  one  which  is  not  easy  to 
follow  on  account  of  the  varying  sizes  in  pole  tops,  is  to  bore 
no  hole  whatever  through  the  pole,  but  to  attach  the  cross-arm 
by  means  of  a  U-bolt  extending  through  the  cross-arm  and 
around  the  pole  and  secured  from  the  front  by  means  of  two 
nuts.  This  means  of  attaching  is  often  used  in  the  case  of  sawn 
poles  where  the  tops  are  of  uniform  size. 

The  arm  is  further  braced  in  any  case  by  the  use  of  wrought- 
iron  or  steel  strips,  commonly  termed  cross-arm  braces.  These 
should  consist  of  straight,  flat  bars  not  smaller  than  ij*  inch 
wide  by  J-  inch  thick,  and  varying  in  length  from  20  to  30 
inches.  A  hole  is  usually  punched  in  one  end  for  the  reception 
of  a  £-inch  or  f-inch  lag-screw  and  in  the  other  for  a  -f-inch 
carriage  bolt.  The  two  braces  for  each  cross-arm  are  attached 
by  single  lag-screws  to  the  pole  at  a  distance  varying  from 
16  to  1 8  inches  from  the  bottom  of  the  arm.  The  other  ends 
of  the  braces  are  attached  by  carriage  bolts  to  the  cross-arms  at 
points  about  equal  distances  from  the  pole.  In  all  cases  suitable 
washers  should  be  used  under  carriage  bolt  nuts  and  heads,  and 
under  "lag-screw  heads  where  they  are  used  in  attaching  an  arm 
to  the  pole.  All  hardware  to  be  used  on  poles,  such  as  bolts, 
washers,  braces,  etc.,  should  be  thoroughly  galvanized  and  should 
be  made  to  stand  the  same  test  that  is  required  on  galvanized 
iron  wire — that  is,  four  successive  plunges  of  seventy  seconds 
each  in  a  saturated  solution  of  sulphate  of  copper  without 
removing  all  of  the  zinc  coating.  The  pins  most  commonly 
used  are  of  locust  or  of  oak.  The  former  is  by  far  the  better,  as 
it  is  the  stronger  and  more  capable  of  resisting  the  action  of  the 
weather.  It  is,  however,  nearly  twice  as  expensive  as  oak.  The 
pins  should  be  turned  from  split  wood  in  order  that  they  may 
not  b£  cross-grained,  and  all  pins  should  be  given  two  coats  of 
the  same  kind  of  paint  that  is  used  on  cross-arms. 

In  some  cases  on  corners,  or  in  places  where  excessively  heavy 


POLE-LINE    CONSTRUCTION.  367 

strain  will  be  brought  upon  a  pin,  it  is  advisable  to  use  a  wrought- 
iron  or  steel  pin,  but  these  must  be  used  with  caution,  as  in 
many  cases  they  have  proven  inferior  to  wooden  pins,  being  so 
soft  that  they  bend  into  a  horizontal  position  when  subjected  to 
the  strain. 

The  insulators  used  in  this  country  are  universally  made  of 
glass.  Blown  glass  has  been  found  to  be  much  superior  in  insu- 
lating qualities  to  molded  glass,  but  the  latter  is  so  very  much 
cheaper  that  it  is  always  furnished.  Fig.  273  shows  a  form  of 


Figs.  273  and  274. — Pony  and  Double-Petticoat  Glass  Insulators. 

insulator  largely  used  in  telephone  work,  called  the  "  pony " 
insulator,  and  Fig.  274  shows  another  style,  termed  the  "  double- 
petticoat  "  insulator.  It  is  so  termed  from  the  fact  that  it  has 
two  lower  flanges,  as  shown  in  section,  the  idea  of  this  being  that 
the  path  for  leakage  from  the  line  to  the  pin  is  thereby  rendered 
considerably  longer,  the  leakage  current  having  to  pass  up  and 
down  the  surfaces  of  both  petticoats  in  series. 

Glass  is  not  as  suitable  a  material  for  insulators  as  porcelain, 
which  is  largely  used  in  Europe.  It  is  more  brittle  and  does  not 
possess  such  high  insulating  qualities.  A  more  serious  defect  is, 
that  it  gathers  moisture  on  its  surface  to  a  much  larger  extent 
than  porcelain,  thus  affording  a  better  path  for  leakage  currents. 
In  an  interesting  series  of  experiments  described  by  Abbott  it 
was  found  that  the  insulating  quality  of  glass  insulators  varied 
largely  with  the  condition  of  the  surface  of  the  insulators. 
These  experiments  were  conducted  over  a  period  of  one  hundred 
and  fifty  days,  observations  being  made  once  a  day.  The  gen- 
eral result  indicated  that  the  greatest  loss  in  insulation  occurred 
during  foggy  or  misty  weather.  During  heavy  rainstorms  the 
insulation  was  somewhat  higher,  and  after  the  storm,  when  the 
insulators  had  been  dried,  the  resistance  of  the  line  was  con- 


368  AMERICAN    TELEPHONE  PRACTICE. 

siderably  higher,  owing  to  the  cleaner  condition  of  the  surface. 
In  good  weather  the  double-petticoat  insulators  gave  much 
higher  resistance  than  the  single  of  corresponding  size,  but  dur- 
ing a  rainstorm  the  double-petticoat  form  was  inferior  to  the 
single,  although  it  was  found  to  dry  more  rapidly  after  a  storm. 

The  determination  of  the  pole-line  route  is  a  matter  of  no 
small  importance.  Right  of  way  must  be  secured,  and  this 
usually  calls  forth  all  the  ingenuity  of  the  party  unfortunate 
enough  to  be  assigned  to  that  duty.  Before  distributing  the 
poles  and  other  material  the  route  should  be  thoroughly  studied 
in  every  detail.  Stakes  should  be  driven  marking  the  location 
of  the  poles.  It  should  be  borne  in  mind  in  locating  these  stakes- 
that  bends  in  the  pole  line  should  be  avoided  wherever  possible, 
that  the  ground  should  be  of  such  nature  as  to  form  as  good 
a  support  as  possible  for  the  pole,  that  there  will  be  no  inter- 
ference from  trees,  houses,  or  other  poles,  and  lastly  that  the 
route  shall  be  as  direct  as  possible.  When  a  turn  must  be  made 
it  should  be  so  located  if  possible  that  the  guy  wire  required  to 
hold  up  the  corner  will  have  suitable  anchoring  ground.  Lack 
of  attention  to  these  preliminary  details  too.  often  brings  an 
endless  amount  of  trouble  in  the  way  of  rehandling  of  poles, 
redigging  of  holes,  and  similar  useless  labor. 

When  the  ground  is  level,  or  gently  undulating,  no  provision 
need  be  made  for  grading  the  pole  tops.  Where,  however,  the 
country  is  hilly  it  is  well  to  make  a  survey  of  the  route  with  a 
level,  placing  the  instrument  between  each  successive  pair  of 
stakes  and  taking  a  front  and  back  sight  from  each  position  to 
the  adjacent  stakes.  A  record  of  the  data  thus  obtained  will 
enable  one  to  plat  the  vertical  section  of  the  route.  The  profile 
of  the  pole  tops  may  then  be  platted,  care  being  taken  to 
smooth  out  all  sharp  bends  in  it.  This  is  accomplished  by 
putting  the  tallest  poles  in  the  hollows  and  the  shortest  on  the 
hilltops.  The  same  results  may  be  accomplished,  though  not  so 
well,  without  the  use  of  the  level,  but  it  requires  an  experienced 
eye  to  do  it  to  best  advantage. 

After  having  decided  on  the  location  of  the  poles,  the  length 
of  pole  for  each  point,  and  all  other  preliminary  details,  such  as 
placing  of  heavy  poles  at  the  corners,  the  poles  may  be  hauled 
and  distributed  along  the  route.  They  should  be  laid  with  the 
butt  near  the  stakes  and  pointing  downhill  if  on  a  grade. 

The  poles  are  distributed  along  the  route  by  any  available 
means.  If  the  line  runs  along  a  railroad,  they  may  be  rolled 
from  the  flat  car  at  the  proper  intervals,  and  carried  to  their 


P  OLE-LINE    CONS  T£  UC  7  70 Ar. 


369 


places  by    carry-hooks   (Fig.  275).     If  the  line  is  a  long  one,  and 
does  not   follow  the  line  of  a  railroad,    the  poles  should  be  un- 


Fig-  275.— Carry  Hook. 

loaded   from  the  cars  at  convenient  points,  and  hauled  to  their 
proper  locations  by  wagons. 

The  cutting  of  gains  and  the  peaking  of  the  pole  may  be  facili- 
tated  by  the  use  of  a  template,  shown  in  Fig.  276,  by  which  the 


Fig.  276. — Gaining  Template. 

gains  and  peak  may  be  marked  out.     A  pole-buck,  constructed 
as  shown  in  Figs.  277  and  278,  and  used  as  in  Fig.  279,*  will  also  be 


'RON  WASHER 


3/l<t    lnON  CHAIN 

Figs.  277  and  278.— Pole  Buck. 


of  great  aid  in  the  work.  The  spacing  between  the  gains,  shown 
on  the  template  in  Fig.  276,  makes  the  distance  between  the 
cross-arms  18  inches.  Many  construction  men  prefer  20  inches. 

*For  the  half-tones  and  some  of  the  detailed  cuts  of  construction  tools  in  this  chap- 
ter we  are  indebted  to  an  excellent  article  in  the  American  Electrician,  by  Mr.  S.  H. 
Dailey,  entitled  "  Erecting  a  High- Voltage  Transmission  Line." 


37° 


AMERICAN    TELEPHONE   PRACTICE. 


Where  a  greater  number  of  arms  are  to  be  used  the  distance  from 

the  top  of  the  top  arm  to  the  peak  should  be  reduced  to  10  inches. 

Poles  of  medium  length  may,  under  ordinary  circumstances,  be 

raised  with  the  cross-arms  in  place,  and,  as  they  are  much  more 


Fig.  279. — Gaining  Poles. 

easily  attached  on  the  ground,  this  should  always  be  done  where 
possible. 

In  digging  the  pole   holes  long-handled  digging  shovels   (Fig, 
280)  and   spoon  shovels  (Fig.  281)   having  seven- and  eight-foot 


Fig.  280.— Long  Handle  Digging  Shovel. 

handles    are  used   in   conjunction   with   eight-foot  steel   digging 
bars,  shown  in  Fig.  282.     Sometimes  the  post-hole  auger  is  used, 


Fig.  281. — Spoon  Shovel. 

but  this  is  only  where  the  conditions  are  very  favorable.     Dyna- 
mite, judiciously  applied,  is  now  being  used  successfully  in  digging 


Fig.  282. — Digging  Bar. 

holes,  even  where  the  soil  is  of  such  a  nature  as  not  to  absolutely 
require  its  use. 


POLE-LINE   CONSTRUCTION.  371 

No  definite  rule  can  be  given  for  the  depth  at  which  poles 
should  be  set  in  the  ground.  The  character  of  the  soil,  the  dis- 
tance between  poles,  the  number  of  wires  carried,  and  the 
sharpness  of  the  turns  made  in  the  line  must  all  be  considered  in 
determining  this  question.  For  average  work  the  data  given  in 
Table  XL  are  believed  to  be  in  accordance  with  the  best  practice. 

TABLE    XI. 

25-foot  pole,  5-J-  feet  in  ground. 

30  "  "  6  "  " 

35  "  "  6  "  " 

40  "  "  6  "  " 

45  "  "  6|  "  " 

50  "  "  6J  "  " 

55  "  "  6|  "  " 

60  "  "  7  u  " 

65  "  «  7  -  « 

70  "  "  7|  "  " 

On  curves  or  corners  the  holes  should  be  dug  from  six  inches 
to  one  foot  deeper  than  is  specified  in  this  table. 

After  digging  the  holes  the  poles  are  carried  or  rolled  by  cant- 
hooks  (Fig.  283),  so  that  its  butt  is  over  the  hole.  A  piece  of 


,IK' 


fz:          1^i-_r*±7 

Fig.  283.— Cant  Hook. 

scantling,  or,  preferably,  a  hardwood  board  in  the  form  of  a 
large  paddle,  is  placed  in  the  hole  to  serve  as  a  rest  for  the 
butt  of  the  pole  while  it  is  being  raised.  The  use  of  this  paddle 
prevents  the  crumbling  of  the  earth  which  is  sure  to  result  and 
cause  much  trouble  if  this  precaution  is  not  taken. 

The   tools   required   in   raising   poles   of  the  average  length— 
from  30  to  50   feet — are  five   or  six   pike-poles  (Fig.  284),  with 


Fig.  284.— Pike  Pole. 

handles  ranging  from  12  to  16  feet  in  length,  and  two  dead  men 
or  pole  supports,  shown  in  Fig.  285. 


372 


AMERICAN   TELEPHONE   PRACTICE. 


The  pole  is  raised  slightly  and  its  end  slipped  into  the  hole, 
resting  all  the  while  against  the  paddle  or  scantling.     The  small 


Fig.  285.— Dead  Man. 

end  of  the  pole  is  then  raised  higher  and  the  dead  men  placed 
under  it,  while  the  men  obtain  another  hold.     The  pole  is  raised 


Fig.  286. — Tamping  Bar. 

gradually,  the  support  being  each  time  moved  closer  to  the  butt. 
When  too  high   to   be  handled   directly,  the  pike-poles  are  used 


Fig.  287. — Raising  Pole. 

on  its  upper  part  (Figs.  287  and  288),  and  in  this  way  it  is  readily 
raised  into  a  vertical  position,  slipping  into  the  hole  while  bearing 


POLE-LIXE   CONSTRUCTION.  373 

against  the  paddle.  It  is  then  braced  by  the  pike-poles,  as 
shown  in  Fig.  289,  and  turned  by  means  of  cant-hooks,  so  that 
the  gains  or  cross-arms,  if  they  were  attached  before  raising  the 
pole,  are  in  proper  position  ;  it  being  remembered  that  the  cross 
arms  should  face  each  other  on  every  alternate  pair  of  poles. 
The  hole  is  then  filled  in  with(  the  soil  which  was  removed  from 
it  in  digging,  the  soil  being  thoroughly  tamped  with  tamping 
bars,  shown  in  Fig.  286,  from  the  bottom  up.  Great  care  should 


m 

afe 


i 


Fig.  288.— Raising  Pole. 

be  taken  that  the  shoveling  in  is  not  done  so  fast  that  the  earth 
cannot  be  properly  tamped.  This  is  frequently  the  cause  of 
much  trouble,  and,  while  it  greatly  expedites  the  erecting  of  the 
poles,  it  causes  much  loss  of  time  and  money  later,  on  account 
of  the  poles  giving  way  when  placed  under  strain.  If  the  soil  is 
soft  a  foot-plate  should  be  placed  under  the  butt  of  the  pole. 
This  can  be  made  by  fastening  together  two  2"  x  12"  pieces  of  oak 
or  hard  pine,  2  or  2\  feet  long,  at  right  angles  to  each  other.  In 
case  the  soil  is  very  soft,  as  in  marshy  districts,  more  elaborate 
means  will  have  to  be  taken.  The  hole  should  be  dug  in  such 
places  much  larger  than  in  ordinary  instances,  and  a  larger  foot- 
plate may  be  inserted.  A  good  plan,  under  these  conditions, 
is  to  place  in  the  bottom  of  the  hole  a  layer,  6  inches  deep,  of 


374  AMERICAN   TELEPHONE   PRACTICE. 

concrete,  and,  after  raising  the  pole,  filling  in  the  entire  hole  to 
the  surface  of  the  ground  with  the  same  mixture,  thoroughly 
tamped  into  place.  For  this  purpose,  and  for  other  cases  where 


Fig.  289.— Pole  Raised. 

concrete  is  needed  in  line-construction  work,  the  following  for- 
mulas are  given : 

FORMULA    NO.   I. 

Natural  Cement,  i  part. 

Sand,  .          .  .         .         .  2    " 

Broken  Stone,  .         .         .  3    " 

FORMULA  NO..  2. 

Portland  Cement,  i  part. 

Sand, 3     " 

Broken  Stone,  .         .         .  7     " 

FORMULA    NO.  3. 

Portland  Cement,  i  part. 

Sand,  .         .  0         .          .  2\  " 

Gravel,     .          .  3     " 

Broken  Stone,     .  .  5     " 


POLE-LINE    CONSTRUCTION.  375 

These  three  formulas  are  all  good,  and  the  one  may  be  used 
for  which  the  material  may  be  most  readily  obtained  in  the  par- 
ticular location  in  question.  Broken  stone  is,  as  a  rule,  better 
than  gravel,  and  stones  of  varying  size,  up  to  the  size  of  an  egg, 
are  somewhat  cheaper  than  stones  of  uniform  size,  because 
the  small  stones  fill  in  the  interstices  between  the  large  ones,  and 
thus  require  less  cement,  while  the  concrete  is  just  as  strong. 

On  a  straight  line  three  different  kinds  of  strain  must  be  pro- 
vided for,  namely  :  the  crushing  strain,  due  to  the  weight  of 
the  wires  ;  the  side  strain,  due  to  wind  pressure  ;  and  the  strain 
in  the  direction  of  the  wires.  This  latter  is  due  to  the  tension  in 
the  wires  at  the  end  of  the  line,  or  to  wind  pressure  in  the  direc- 
tion of  the  line,  or  to  the  tension  in  portions  of  the  line  caused 
by  the  falling  of  a  pole  or  the  breaking  of  a  number  of  wires. 
In  hilly  country  also  considerable  strain  is  caused  in  the  direc- 
tion of  the  line  itself  on  a  long  down  grade,  due  to  the  actual 
weight  of  the  wires.  The  first  two  strains,  that  is,  the  crushing 
strain  and  the  side  strain  due  to  wind,  are  at  times  very  great, 
both  being  augmented  by  the  formation  of  a  crust  of  ice  on  the 
wires  and  poles  during  sleet  storms.  Abbott  cites  cases  where 
coatings  of  ice  six  inches  in  diameter  have  been  formed  on  a  No. 
10  wire  throughout  its  length.  These,  of  course,  are  extreme 
cases,  but  coatings  two  inches  in  diameter  are  quite  common.  It 
is  customary  to  provide  for  the  crushing  and  side  strains  on  a 
straight  line  by  making  the  poles  heavy  enough  to  stand  them 
without  recourse  to  other  methods,  although  on  very  heavy  lines 
side  guys  are  often  used,  even  on  straightaway  work.  The 
sizes  of  poles  given  in  table  on  page  361  is  sufficient  to  insure 
against  breakage  in  such  cases  under  all  ordinary  conditions. 

The  strain  in  the  direction  of  the  wires  should  be  provided 
for  by  a  method  of  bracing  known  as  head-guying.  This  con- 
sists in  running  a  guy  wire  from  the  base  of  one  pole  close  to 
the  ground  to  the  top  of  the  next,  etc.,  for  several  poles  in  suc- 
cession. About  three  poles  should  be  guyed  from  the  top  of 
one  to  the  butt  of  the  next,  and  in  the  next  three  the  order  should 
be  reversed,  thus  bracing  the  line  in  both  directions.  This,  if 
repeated  at  intervals  of  one  mile,  will  greatly  strengthen  the 
line  against  vibration  in  the  longitudinal  directions  caused  by 
high  winds  or  by  the  other  causes  mentioned.  On  a  down  grade 
the  head-guys  should  extend  from  the  butt  of  the  pole  on  the 
highest  ground  to  the  top  of  the  pole  below  it.  The  method  of 
head-guying  is  illustrated  in  Fig.  290.  When  a  line  is  dead- 
ended  at  the  termination  of  a  lead,  or  for  the  purpose  of  con- 


376 


AMERICAN    TELEPHONE   PRACTICE. 


necting  with  an   underground  cable,  the  last  three  poles  should 
be  head-guyed   by   running  a  guy  wire  from  the  bottom  of  the 

H -*• 


Fig.  290. — Head  Guying. 

last  pole  to  the  top  of  the  next,  and  so  on  for  three  poles.     The 
last  pole    should   be  guyed  by  planting  a  guy-stub  at  as  great  a 


Fig.  291. — Terminal  Pole. 

distance  as  possible  beyond  it,  in  the  direct  line  of  the  poles  and 
firmly  guying  to  it.  It  frequently  happens  in  cities  that  suffi- 
cient room  cannot  be  obtained  for  dead-ending  a  pole  line  in  this 


POLE-LINE    CONSTR UCTION. 


377 


manner,  and  under  these  conditions  some  sort  of  an  anchor  pole 
is  necessary.  Frequently  room  may  be  had  by  planting  the 
anchor  at  a  distance  of  perhaps  ten  feet  from  the  base  of  the 
pole,  as  shown  in  Fig.  291.  In  this  case  the  guy  wire  or  rod  should 
be  made  very  strong,  in  order  to  successfully  stand  the  excessive 


292.— Details  of  Anchor  Pole,  and  Guy  Rod. 


strain,  and  the  anchor  should  be  buried  to  a  depth  of  perhaps  eight 
feet,  and  weighed  down  by  a  mass  of  rock  and  concrete.  As  an  addi- 
tional precaution  a  lattice-work  of  angle  iron  is  in  some  cases  used 
to  re-enforce  the  upper  portion  of  the  pole  for  the  purpose  of 
equalizing  the  pull  on  the  guy  rod  without  undue  stress  on  any 
portion  of  the  pole.  In  Fig.  292  is  shown  such  a  lattice-work, 
and  also  a  good  method  of  anchoring  a  pole  to  be  subjected  to  a 
severe  strain.  Structural  iron  anchor  poles  are  sometimes  used 
for  the  termination  of  very  heavy  leads,  and  these  offer  the  neat- 
est solution  of  the  problem,  but  have  the  disadvantage  of  being 
extremely  expensive. 


378 


AMERICAN    TELEPHONE   PRACTICE. 


When  a  bend  occurs  in  the  line  or  when  a  heavy  branch  lead 
is  taken  off  at  an  angle,  a  very  severe  side  strain  is  exerted  on  the 
poles.  These  strains  must  be  amply  provided  for  by  means  of  a 
system  of  braces  which  are  capable  of  exerting  an  opposite  pres- 
sure to  that  of  the  pull  of  the  wires.  This  is  usually  done  by 
means  of  guy  wires,  connected  to  the  tops  of  the  poles  and  ex- 
tending in  such  direction  as  to  bisect  the  angle  of  the  bend  which 
the  line  makes.  On  long  curves  a  guy  wire  should  be  provided 
for  each  pole,  and  it  is  also  well  to  head-guy  each  pole.  Begin- 
ning at  the  center  of  the  curve,  head-guys  should  extend  from 


Fig.  293. — Y-Guying. 

the  base  of  each  pole  to  the  top  of  the  next  pole  in  each  direc- 
tion from  the  center.  The  shorter  the  turn  the  greater  the 
strain,  and  the  greater,  therefore,  must  be  the  precaution  taken 
to  meet  it.  The  best  method  of  side  guying  is  known  as  the  Y- 
guy,  shown  in  Fig.  293.  Where  more  than  four  cross-arms  are 
used  a  Y-guy  should  always  be  employed,  as  it  takes  the  strain 
from  both  the  top  and  bottom  arm.  To  guy  from  the  top  of 
the  pole  only,  as  is  frequently  done,  causes  the  latter  to  bow 
toward  the  center  of  the  curve  at  the  lower  cross-arm,  and  fre- 
quently causes  the  pole  to  break  at  that  point,  usually  in  the 
gain  of  the  lower  arm.  On  the  other  hand,  to  guy  from  the 
lower  cross-arm  usually  causes  a  pole  to  bow  in  the  other  direc- 
tion with  the  same  result. 


POLE-LINE    CONSTRUCTION. 


379 


In  turning  a  sharp  corner,  as,  for  instance,  the  corner  of  a 
street,  it  is  better  to  use  two  poles,  which  may  equally  stand  the 
strain.  Such  a  plan  is  shown  in  Fig.  294.  The  wires  of  the 


c 


Head  Guy 


Fig.  294. — Double  Pole  Corner. 

bend  are  somewhat  closer  together  than  those  on  the  straight 
portions  of  the  line,  but  this  is  a  matter  of  almost  no  impor- 
tance, and  could  easily  be  obviated  by  making  the  cross-arms  of 


Fig.  295. — Guy  Anchor. 

these  two  poles  somewhat  longer.  These  two  poles  should,  if 
possible,  be  guyed  in  a  manner  which  will  effectually  brace  them 
in  all  directions. 


38o 


AMERICAN    TELEPHONE   PRACTICE. 


To  properly  anchor  guy  wires  often  requires  a  good  deal  of 
ingenuity,  and  it  is  hard  to  lay  down  any  definite  rules,  as  they 
frequently  have  to  be  planned  to  meet  the  existing  conditions. 
One  of  the  most  common  methods,  and  a  very  satisfactory  one,  is 
shown  in  Fig.  295.  The  anchor  log  should  be  not  less  than  ten 
inches  in  diameter,  and  from  four  to  six  feet  long.  A  railroad 
tie  is  an  excellent  thing  for  this  purpose.  The  anchor  rod  is 
usually  of  wrought  iron,  from  six  to  eight  feet  long,  and  from 
|  to  I  \  inch  in  diameter,  having  an  eye  forged  in  one  end  and  a 
heavy  screw  thread  and  nut  on  the  other.  The  rod  should  pass 


Fig.  296. — Guy  Stub  and  Anchor. 

directly  through  the  anchor  log  and  be  secured  by  the  nut,  a 
heavy  iron  washer  being  placed  between  the  log  and  the  nut. 
All  iron  wire  so  used  should  be  galvanized  and  subject  to  the 
same  test  as  that  required  for  galvanized  iron  wire.  Where  a 
particularly  heavy  strain  is  to  come  on  an  anchor  log  it  is  well  to 
place  heavy  two-inch  planks  over  the  log,  and  at  right  angles 
to  the  guy  rod,  and  above  these  heavy  stones  may  be  placed. 
In  extreme  cases  the  log  should  be  buried  in  a  mass  of  concrete. 
Another  very  common  way  of  attaching  a  guy  wire  is  to  a 
guy-stub,  which  is  usually  formed  of  the  stub  end  of  a  pole  from 
8  to  12  feet  long,  set  from  6  to  8  feet  in  the  ground,  at  an  angle 
of  approximately  90  degrees  to  the  direction  of  the  guy  wire. 


POLE-LINE    CONSTRUCTION.  381 

Where  this  construction  is  used  the  guy  should  be  attached  to 
the  stub  as  close  to  the  ground  as  possible,  and  never  at  a 
greater  distance  than  three  feet  from  the  ground,  except  where 
additional  precautions  are  taken  to  anchor  the  stub  itself.  Where 
it  is  necessary  in  crossing  a  road  with  a  guy  wire  to  raise  the 
wire  to  a  greater  height  from  the  ground  than  this  construction 
would  allow  a  longer  guy-stub  should  be  used,  as  illustrated  in 
Fig.  296,  and  this  should  be  anchored  as  shown  in  Fig.  295. 

Still  another  method  of  providing  against  side  strain  is  by  the 
use  of  a  pole  brace.  The  pole  brace  should  conform  to  the 
same  specifications  as  the  regular  poles  used,  and  is  placed  as  a 
prop,  usually  making  an  angle  of  about  30  degrees  with  the  pole 
itself.  It  is  placed  always,  of  course,  on  the  inside  of  the  curve, 
and  in  such  direction  as  to  bisect  the  angle  of  the  wires  at  that 
point.  The  top  of  the  brace  should  be  chamfered  and  secured 
to  the  pole  by  several  twists  of  heavy  guy  wire,  and  may  be 
further  held  from  slipping  by  the  insertion  of  lag-screws  or  spikes. 
The  bottom  of  the  pole-brace  should  be  inserted  into  the  ground 
to  a  distance  of  at  least  three  feet,  and  should  rest  on  a  suitable 
butt  plate  if  the  ground  is  at  all  soft. 

The  guy  rope  should  be  fastened  to  the  pole  by  passing  it 
twice  around  and  clamping  it  by  means  of  some  such  malleable- 
iron  guy  clamp  as  is  shown  in  Fig.  297.  If  there  is  any  possi- 


Fig.  297. — Guy  Clamp. 

bility  of  the  guy  wire  slipping  on  the  pole  this  may  be  prevented 
by  securing  it  with  staples.  Care  should  be  taken,  however,  in 
the  driving  of  the  staples  not  to  injure  the  guy  wire  by  kink- 
ing it. 

The  wire  used  in  guying  may  consist  of  one  or  more  strands 
of  No.  9  or  10  B.  &  S.  steel  wire  twisted  together,  but  a  better 
plan  is  to  use  the  regular  steel  cables,  thoroughly  galvanized, 
furnished  by  the  several  reliable  wire  manufacturers.  This  has 
the  advantage  of  being  more  flexible,  more  easily  handled,  and, 
at  the  same  time,  stronger  for  its  weight  than  the  single  strands 


382  AMERICAN   TELEPHONE  PRACTICE. 

of  larger  wire.      The  cable  should  consist  of  seven  No.  12  steel 
wires  laid  up  with  a  3j-inch  twist. 

THE   STRINGING   OF   WIRES. 

After  about  a  mile  of  poles  have  been  set  and  guyed,  and  the 
cross-arms,  pins,  and  insulators  put  in  place,  the  process  of 
stringing,  where  but  a  few  wires  are  to  be  run,  consists  in  plac- 
ing the  reels  on  hand  barrows,  as  is  shown  in  Fig.  298,  or  on  a 


Fig.  298. — Hand  Barrow. 

cart,  and  paying  them  as  they  go,  drawing  the  wire  up  to 
each  pole  separately.  When,  however,  a  larger  number  of  wires 
are  to  be  run  the  method  is  briefly  as  follows:  The  separate 
coils  of  wire  are  placed  on  spindles  at  the  beginning  of  the 
stretch  to  be  strung  and  each  is  attached  to  a  hole  in  a  "  running 
board,"  which  is  of  about  the  same  dimensions  and  has  the 
same  spacing  as  a  cross-arm.  To  the  center  of  this  running 
board  a  "  running  rope  "  is  attached — and  this  is  placed  on  the 
top  of  all  the  cross-arms  in  the  stretch.  A  team  of  horses 
hitched  to  the  other  end  of  the  rope  then  "walk  away" 
with  it.  A  man  is  stationed  on  each  pole  in  order  to  lift  the  run- 
ning board  over  the  top  of  each  pole  or  to  properly  guide  it 
around.  After  the  wires  are  all  in  place  each  one  is  separately 
pulled  up  to  the  proper  tension,  and  at  a  given  signal  is  tied  to 
the  insulator  at  each  pole. 

Two  distinct  methods  are  used  for  securing  proper  tension. 
In  each  case  the  force  is  applied  by  attaching  a  wire  clamp, 
commonly  known  as  a  "  come-along,"  shown  in  Fig.  299,  and 
pulling  it  up  with  a  block-and-tackle  or  by  hand.  In  one  method 
the  proper  degree  of  tension  is  obtained  by  the  use  of  the  dy- 
namometer, which  is  merely  a  form  of  spring  balance.  The 
tension  depends  on  the  kind  and  size  of  wire,  on  the  distance 


POL E-LINE   CONS  TR  UC  7 'ION. 


383 


between  the  poles,  and  on  the  temperature  at  time  of  the  string- 
ing. The  amount  of  tension  put  on  each  wire  is  usually  taken 
as  about  one-third  the  breaking  strength  of  the  wire,  which  may 
be  found  from  the  wire  tables.  The  other  method  is  to  allow  a 
certain  sag  or  distance  between  the  center  of  the  span  and  the 
straight  line  between  the  points' of  support.  Table  XII.,  which 


Fig.  299. — Come-along. 

is  taken  from  Roebling's  handbook  on  "  Wire  in  Electrical 
Construction,"  gives  the  sag  in  inches  for  the  various  lengths  of 
span  at  different  temperatures,  these  figures  being  based  on  the 
use  of  good  hard-drawn  copper  wire. 


TABLE  XII. 

AMOUNT  OF  SAG  IN  SPANS. 


•21 

Spans  in  Feet. 

11- 

75 

100 

II5 

130 

150 

200 

s  «*§ 

Sag  in  Inches. 

-30 

i 

21 

31 

4i 

8 

-10 

JT 

3 

3f 

5 

9 

10 

Ii 

2% 

3i 

4f 

5t 

IOT 

30 

if 

3 

4 

sl 

6f 

12 

60 

2* 

4i 

5i 

7 

9 

I5| 

80 

3* 

5f 

7 

H 

i8f 

100 

4* 

7 

9 

11 

14 

22^ 

It  is  the  practice  of  a  certain  company  using  forty  poles  to  the 
mile  to  allow  on  either  copper  or  iron  wire  a  three-inch  dip  or 
sag  at  the  center  of  spans  in  the  eastern  portion  of  the  United 


384  AMERICAN    TELEPHONE  PRACTICE. 

States  and  an  eight-inch  sag  in  the  western  portion.  The  reason 
for  the  difference  in  the  allowable  sag  in  the  East  and  in  the 
West  is  due  to  the  fact  that  far  greater  variations  in  temperature 
occur  in  the  West  than  in  the  East. 

Two   patterns   of  climbers  are   in  general  use,  known  respec- 
tively as    the   Eastern  and  the  Western  climbers.     In  the  East- 


Fig.  300. — "  Western  "  Climber.     Fig.  301. — "  Eastern"  Climber. 

ern  the  strap-bar  passes  up  the  inside  of  the  leg,  and  in  the  West- 
ern it  is  on  the  outside.  These  are  shown  in  Figs.  300  and  301. 
The  tying  of  wires  to  the  insulators  is  an  important  matter, 
and  there  are  several  different  methods  of  doing  it.  The  ordi- 
nary method,  used  almost  since  the  beginning  of  line  construc- 
tion, is  shown  in  Fig.  302.  In  this  the  line  wire  merely  passes 


Fig.  302.— Ordinary  Tie. 

along  the  side  of  the  insulator  and  is  held  in  the  groove  by  a 
tie  wire,  twisted  around  the  line  wire  at  each  end  as  shown. 
The  tie  wires  are,  as  a  rule,  about  sixteen  inches  long,  and  made 
of  slightly  smaller  diameter  than  the  line  wire  itself,  especially 
in  cases  of  very  heavy  wire. 

Another  method,  known  as  the  Helvin  tie,  is  shown   in  Fig. 
363.     This  has  been   used  with  considerable  success  with  hard- 


POL E-LINE   CONS  TR  UC TION. 


385 


drawn  copper  wire.  In  this  the  tie  wire  is  first  wrapped  around 
the  insulator  and  twisted  once  or  twice  on  itself,  after  which  the 
ends  are  twisted  around  the  line  wire  as  before. 

Still   another  method  of   tying  the  wire  to    the    insulator  is 
shown    in    Fig.    304.      In    this,    as    in    the    first     method,    the 


Fig-  303.— Helvin  Tie. 

line  wire  is  laid    in   the    groove   of   the    insulator 

wire    is    passed     entirely   around    the     groove, 

ing  down  over  the  line  and  the  other  end  up  under  it,  the  twist 

being  made  as  shown.     This  tie  is  perhaps  the  best  of  all  where 

properly  made,  and  is  now  much  used  in  telephone  work.    Where 

a   wire  is  dead-ended   it  is  simply  passed    once  around  the  insu- 


and  the   tie 
one    end    pass- 


Fig.  304. — Latest  Method  of  Tying. 

lator  and  twisted  several  times  upon  itself,  the  twist  beginning  at 
a  distance  of  about  two  inches  from  the  insulator.  In  the  case 
where  transpositions  are  to  be  made  the  free  end  of  the  wire 
should  be  left  long  enough  to  pass  over  and  make  connection 
with  the  other  side  of  the  circuit. 

The  joining  of  wires  is  a  matter  which  has  received  much  at- 
tention. The  old  style  of  joint,  and  one  which  gives  much  satis- 
faction, is  shown  in  Fig.  305.  This  is  known  as  the  Western 


386 


AMERICAN    TELEPHONE   PRACTICE. 


Union  joint,  and  is  made  by  placing  the  two  ends  side  by  side 
and  clamping  them  with  a  hand  vise  or  with  a  heavy  pair 
of  pliers.  With  another  pair  of  pliers,  held  in  the  right  hand, 


Fig-  305.— Western  Union  Wire  Joint. 

the  free  end   of   each  wire  is  twisted   tightly  around  the  other 
wire,  as  shown. 

Another  method  of  joining  wires,  known  as  the  Mclntire  sleeve 
joint,  is  shown   in    Fig.   306.      The  sleeve  for  making  this  joint 


Fig.  306. — Mclntire  Sleeve  Joint. 

consists  of  two  copper  tubes  soldered  together  and  having  a  bore 
corresponding  to  the  sizes  of  the  wire  to  be  joined.  The  ends 
of  the  wire  are  passed  in  opposite  directions  through  these  tubes 
and  are  then  grasped  at  each  end  with  a  special  tool  for  the  pur- 
pose and  given  three  distinct  twists.  This  joint  is  now  widely 
used  in  practice  and  is  very  convenient  because  the  use  of  solder 
is  not  required  in  order  to  make  it  perfect. 

Still  another  connector,  known  as  the  Lillie  joint,  is  shown  in 
Fig-  307.  The  connector  in  this  consists  in  a  sheet  of  copper 
curved  longitudinally  in  opposite  directions.  The  wires  are 


Fig.  307. — Lillie  Wire  Joint. 

slipped  in  each  curve  of  the  strip  and  twisted  in  opposite  direc- 
tions, as  in  a  Mclntire  joint.  This  joint  has  not  come  into  such 
extensive  use  as  the  Mclntire  sleeve,  but  should  prove  efficient. 
Fig.  308  shows  how  this  sleeve  may  be  applied  in  taking  off 
branch  wires,  as  in  the  case  of  attaching  bridging  telephones  to 
a  line. 


POL E-LINE    CONS  TR  UC  T1ON. 


387 


Practice  differs  somewhat  among  construction  men  as  to  the 
matter  of  soldering  wire  joints,  some  claiming  that  the  solder 


Fig.  308.— Branch  Wires  with  Lillie  Joint. 

joint  gives  no  better  results  either  as  to  conductivity  or  strength 
than  unsoldered  ones. 

The  best  practice,  however,  dictates  the  use  of  solder  on  all 
except  the  patent  sleeve  joints.      In  soldering  a  Western  Union 


Fig.  309. — Transpositions. 

joint,  it  is  well  to  apply  the  heat  only  at  the  center  of  the  splice. 

It  is  sufficient  to  solder  the  joint  at  its  center,  and  the  danger  of 

weakening  the  line  by  the  annealing  effect  of  the  heat  is  reduced. 

The  method   of  making  transpositions  is  shown  in  Fig.  309. 


388 


AMERICAN    TELEPHONE   PRACTICE. 


For  this  purpose  transposition  insulators  having  two  grooves 
may  be  obtained.  In  making  transpositions  a  good,  though  more 
expensive,  way  is  to  use  double  cross-arms  at  the  transposition 
poles,  dead-ending  the  wires  on  each,  and  bridging  across  by 
bridle  wires  in  much  the  same  manner  as  shown. 

It  is  frequently  necessary  to  run  a  telephone  line  on  the  same 
poles  with   a   high-tension   power  circuit.     Induction  from    the 


6~fO  I  Ito     0 


Fig.  310. — Telephone  Line  and  Power  Circuit. 

power  wires  is  of  course  under  these  conditions  very  likely  to 
render  conversation  impossible,  especially  if  the  current  in  the 
power  circuit  is  alternating.  Fig.  310  shows  the  details  of  a 
pole  thus  equipped,  the  two  insulators  on  brackets  being  for  the 
telephone  line.  The  latter  should  be  of  No.  12  B.  &  S. 
copper,  and  transposed  every  three  poles.  In  this  way  a  fairly 
quiet  line  may  be  obtained  under  the  most  unfavorable  circum- 
stances. 


CHAPTER  XXXI. 

( 

OVERHEAD    CABLE    CONSTRUCTION. 

THE  tendency  of  good  telephone  practice  in  cities  is  to  bunch 
the  line  wires  following  the  same  route  into  cables,  and  it  may 
be  added  that  there  is  also  a  strong  tendency  toward  the  placing 
of  these  cables  underground,  this  latter  being  due  in  large 
measure  to  the  protests  of  the  public  against  all  overhead  elec- 
trical construction.  Overhead  cables  are,  however,  used  to  a 
large  extent,  and  there  will  always  be  conditions  under  which 
their  use  will  be  found  advantageous. 

The  overhead  cable  presents  many  advantages  over  the  use 
of  bare  wires.  Besides  the  fact  that  in  many  districts  it  would  be 
absolutely  impossible  to  handle  the  required  number  of  wires 
without  the  use  of  cables,  on  account  of  lack  of  space,  may  be 
mentioned  the  following  :  The  lines  are  rendered  far  more  sightly 
and  offer  much  less  obstruction  to  firemen  in  the  performance 
of  their  duties,  for  two  hundred  or  more  wires,  which  alone,  if  bare, 
would  require  the  use  of  a  pole  line  carrying  at  least  twenty  ten- 
pin  cross-arms,  maybe  crowded  into  a  cylindrical  space  not  over 
two  and  a  half  inches  in  diameter  ;  the  danger  of  crosses  from 
high-tension  or  other  wires  is  greatly  reduced  ;  the  liability  to 
injury  in  heavy  wind  and  snowstorms  is  lessened,  and  the  cost 
of  construction  is  in  many  cases  greatly  cheapened. 

In  regard  to  the  latter  point — comparative  cost  of  construc- 
tion— Table  XIII.,  compiled  by  the  Standard  Undergound  Cable 
Company,  and  based  upon  average  prices  for  material  and  labor, 
is  of  great  interest. 

From  this  it  will  be  seen  that  while  the  bare-wire  construction 
may  be  somewhat  cheaper  for  lines  carrying  fifty  wires  or  less, 
the  cables  have  the  advantage  in  this  respect  when  one  hundred 
or  more  lines  are  carried. 

In  the  early  days  of  telephony  rubber  was  considered  the  best 
insulating  material  for  the  wires  in  cables.  A  cable  so  con- 

o 

structed  is  still  largely  used  by  the  British  post-office  system.  It 
is  constructed  as  follows :  The  conductors  are  each  composed  of 
three  strands  of  tinned  copper  wire,  having  a  size  corresponding 
to  No.  24  B.  &  S.  gauge.  These  three  together  form  a  single 

389 


39° 


AMERICAN   TELEPHONE  PRACTICE. 


TABLE  XIII. 

COMPARATIVE  COST  PER  MILE  OF  OVERHEAD  WIRES  AND  CABLES. 
(SS  Poles  to  the  Mile.} 


Overhead  Wires,  Bare, 
Materials,  etc. 

s'J 

3& 

•Si 

££ 

6  6 
^>^- 

<L>   c/3 

G   <U 

3§ 

| 

%$ 

200-  Wire  Line. 
6o-Foot  Poles. 

Poles  Cedar  

$1-11    2S 

$       l66    25 

$4  c  e   oo 

Poles    Setting 

28  oo 

1  T        CO 

Cross-  Arms  (10  pins) 

6  1  25 

122    50 

Cross-Arms,  attaching  to  poles  
Braces  and  Screws  

17-50 
60 

35.00 
I    2O 

70.00 
2    40 

Pins  (il4  inch  Locust) 

1  7    ^O 

oc    oo 

Pins  attaching  to  arms 

2    60 

5    2O 

10  40 

Insulators.           

21    OO 

42  oo 

84  oo 

Insulators  attaching  to  pins.  .       

I    5O 

o    OO 

6  oo 

No.  14  B.  &  S.  Gauge  Hard  Drawn  Cop- 
per Wire  . 

4.Q7    06 

QQC      Q2 

1087   84 

Labor  Stringing  Wire      

2OO   OO 

380  oo 

740  oo 

Total  

$Q7Q.  l6 

$1817.  «;? 

$3708  64. 

LEAD-COVERED    AERIAL   CABLE. 


Thirty-five  Poles  (30  feet)  

$      52.50 

$         5  2.  5O 

$C2.  ^O 

Labor  Setting 

24.  co 

24.    CO 

24.    ^O 

One  Mile  Galvanized  Strand 

2O.  5Q 

2O   Q 

ei    22 

Stringing  Same,  Including  Supports  .  . 
One-Mile  New  Standard  Cable  and  In- 
stalling Same  Complete 

52.00 
1214.40 

52.00 
1636  80 

52.00 
2428  80 

Total    

$1363.00 

$1786.30 

$2609.02 

conductor  weighing  twenty  pounds  per  mile  and  having  a  resist- 
ance of  45  ohms.  Each  conductor  is  covered  with  two  coats  of 
non-vulcanized  rubber,  after  which  they  are  taped  with  rubber- 
coated  cotton  and  covered  with  ozokerite.  The  wires  are  then 
twisted  together  in  pairs  and  the  required  number  laid  up  into  a 
cable  and  served  with  jute  and  wrapped  with  tape  impregnated 
with  bituminous  compound.  The  whole  core  is  then  again 
coated  with  the  bituminous  compound,  served  with  hemp  soaked 
in  a  compound  of  gas-tar,  and  again  treated  with  the  bituminous 
compound.  It  is  then  served  with  an  external  coating  of  tape 


OVERHEAD    CABLE    CONSTRUCTION. 


39* 


and  a  coating  of  silicated  compound.     This  has  been  found  to  be 
a  reliable  cable  and  thoroughly  water-proof. 

For  short  lengths  rubber-insulated  cable  is  often  used  in  this 
country,  and  under  certain  conditions  is  preferable  to  the  lead- 
covered  paper-insulated  cable  which  will  be  described  later.  The 
three-stranded  conductor,  however,  is  little  used,  a  single  No. 
18  B.  &  S.  guage  tinned  wire  being  used  instead.  These  are 
double-coated  with  rubber  and  separately  tested  in  water  for  in- 
sulation. After  this  they  are  covered  with  braid,  bunched,  and 
the  core  so  formed  covered  with  tarred  jute,  over  which  is  placed 
a  heavy  braid  saturated  with  so-called  weather-proof  compound. 

Table  XIV.,  given  below,  shows  the  sizes  and  weights  of 
the  various  sizes  of  this  cable  as  manufactured  by  a  prominent 
firm  : 

TABLE  XIV. 
AERIAL  CABLE  RUBBER-COVERED  WIRES. 


g 

VM 

<+-*    t/3 

. 

^ 

»-     M 

5-  -2 

£  & 

'~~  c 

0>    3 

|| 

a| 

I1 

3 

5 

10 

15 

6 

10 

20 
30 

i 

175 

256 

452 

633 

20 

40 

't 

813 

25 

50 

it 

994 

Rubber  cables  are  often  incased  in  lead,  in  which  case  the 
rubber  is  made  somewhat  thinner  and  the  braid  over  the  indi- 
vidual wires  and  much  of  that  over  the  entire  bunch  is  omitted, 
because  the  lead  affords  protection  both  from  mechanical  injury 
and  from  the  weather. 

Rubber-covered  cables  give  excellent  results  as  to  insulation 
and  durability  ;  but  a  serious  objection  to  their  use  for  telephone 
work  is  that  their  electrostatic  capacity  is  very  high.  This  is 
due  to  the  fact  that  while  rubber  is  a  splendid  insulator,  its  spe- 
cific inductive  capacity  is  much  higher  than  that  of  some  other 
insulators.  Dry  air  is  the  most  desirable  in  this  respect,  its  spe- 
cific inductive  capacity  being  lower  than  that  of  any  other  known 


392  AMERICAN    TELEPHONE  PRACTICE. 

substance.  A  great  improvement  in  regard  to  the  electrostatic 
capacity  of  cables  has  been  brought  about  by  use  of  paper  insu- 
lation between  the  individual  conductors.  In  the  earlier  forms 
of  cables  so  constructed  the  wires  were  wrapped  with  paper, 
which  was  afterward  impregnated  with  some  insulating  material, 
such  as  paraffin,  having  a  low  specific  inductive  capacity.  It  has 
been  found  by  aerating  the  paraffin  thus  used  with  dry  carbonic 
acid  gas  that  the  electrostatic  capacity  between  the  conductors 
was  reduced  as  much  as  15  per  cent.  In  order  to  still  further 
reduce  the  capacity  what  are  known  as  dry-core  cables  have  been 
introduced  and  have  come  into  extensive  use.  These  are  usually 
formed  by  wrapping  the  separate  conductors  with  two  layers  of 
dry  paper  loosely  laid  on.  Sometimes  only  a  single  wrapping  is 
used.  The  two  wires  which  are  to  form  a  twisted  pair  are,  after 
being  separately  wrapped,  twisted  together,  the  length  of  a  com- 
plete twist  being  about  three  inches.  Another  way  of  forming 
a  twisted  pair  is  to  lay  the  two  wires  upon  opposite  sides  of  a 
strip  of  paper  and  twisting  the  two  together  with  the  paper  be- 
tween them.  The  pair  is  afterwards  served  with  a  single  wrap- 
ping of  paper,  forming  a  complete  tube  around  it.  After  the 
twisted  pairs  are  formed,  by  whatever  method,  the  desired  num- 
ber of  them  are  laid  loosely  together  and  covered  with  a  lead 
sheath,  usually  one-eighth  of  an  inch  in  thickness. 

The  saturated-core  cable  may  be  formed  in  the  same  way,  the 
difference  being  that  the  paper  is  impregnated  with  some  insu- 
lating material  before  the  lead  sheath  is  put  on. 

The  saturated  cable  has  the  advantage  of  not  being  so  suscep- 
tible to  moisture  as  the  dry  core,  but  its  electrostatic  capacity  is 
usually  15  microfarads  per  mile,  or  higher,  while  in  the  dry  core 
capacities  as  low  as  .05  microfarad  are  said  to  have  been  at- 
tained. It  is  doubtful  if  this  latter  figure  could  be  reached  as  an 
average,  and  specifications  for  dry-core  cables  usually  require  an 
average  capacity  of  .080  per  mile.  So  long  as  the  lead  covering 
remains  intact  no  difficulty  is  experienced  with  the  dry-core 
cable,  but  when  a  puncture  is  made  moisture  enters  to  a  sufficient 
extent  to  greatly  lower  the  insulation  of  the  cable.  If  the  dam- 
age is  not  quickly  repaired  a  considerable  length  of  the  cable  is 
apt  to  be  injured,  as  the  moisture  finds  its  way  quickly  through 
the  dry  paper.  For  this  reason,  in  small  telephone  exchanges 
not  equipped  with  the  proper  means  for  testing  out  and  repair- 
ing cables,  the  saturated  core  is  most  desirable.  Where  the 
requisite  means  are  at  hand  for  frequent  testings  the  dry  core  is 
greatly  to  be  preferred. 


OVERHEAD    CABLE   CONSTRUCTION.  393 

The  locating  of  faults  in  cables  may  be  facilitated  by  specify- 
ing that  one  or  two  small  rubber-covered  wires  be  laid  through  the 
center  of  the  cables,  these  wires  afterward  being  reserved  as  test 
wires  for  use  in  the  Varley  loop  test  so  often  used  in  locating 
leaks. 

The  size  of  wire  used  in  telephone  cables  varies  from  No.  18 
B.  &  S.  gauge  to  No.  22,  No.  19  being  probably  the  most  com- 
mon. Specifications  usually  state  that  the  cable  sheath  shall  be 
composed  of  an  alloy  of  lead  and  tin,  the  amount  of  the  latter  be- 
ing not  less  than  three  per  cent,  of  the  entire  mixture.  This  re- 
quirement has  been  made  because  it  has  been  found  that  such  an 
alloy  is  not  so  susceptible  to  chemical  action  as  lead  alone,  an  im- 
portant consideration  in  underground  work.  Much  difficulty  has 
been  found  in  manufacturing  them,  however,  to  secure  an  even 
mixture  of  the  lead  and  tin.  The  Standard  Underground  Cable 
Company  are  firm  advocates  of  the  use  of  a  pure-lead  sheath, 
afterwards  treated  with  an  external  coating  of  pure  tin,  arguing 
that  the  tin  when  mixed  with  the  lead  makes  the  sheath  brittle 
and  that  the  tin  will  be  most  effective  if  all  of  it  is  placed  on  the 
outside.  Notwithstanding  this,  it  is  customary,  as  stated  above, 
to  specify  that  the  sheath  shall  be  composed  of  the  alloy. 

The  use  of  braiding  saturated  with  a  moisture-proof  compound 
placed  over  the  lead  sheath  is  often  advocated.  Opinions  differ 
as  to  the  advisability  of  this,  but  it  is  probable  that  its  disad- 
vantages outweigh  its  advantages  in  either  overhead  or  under- 
ground work.  The  locating  of  punctures  in  the  sheath  is  made 
.much  more  difficult  by  the  use  of  this  braid  in  overhead  cables, 
for  when  the  sheath  is  bare  they  may  be  often  located  by  mere 
external  inspection  ;  moreover,  the  braiding  considerably  in- 
creases the  expense  of  the  cable,  and  its  only  advantage  is  its 
prevention  of  abrasion.  This  need  not  occur  if  the  cable  is  prop- 
erly supported.  In  underground  work  the  braiding  affords  a  pro- 
tection for  the  sheath  during  the  drawing  in  process  and  may 
afford  some  protection  against  chemical  action.  After  it  rots, 
however,  the  pieces  may  so  thoroughly  clog  up  the  conduit  as  to 
prevent  the  withdrawal  of  the  cable,  thus  not  only  losing  that 
length  of  cable,  but  rendering  the  duct  in  the  conduit  unavailable. 

Table  XV.  gives  the  outside  diameter  and  the  weight  per  1000 
feet  of  the  various  sizes  of  lead-covered  paper  cable  manufac- 
tured by  a  prominent  firm.  The  conductors  are  No.  19  B.  &  S., 
-each  being  served  with  two  layers  of  paper. 


394 


AMERICAN    TELEPHONE  PRACTICE. 


TABLE  XV. 

AERIAL  CABLE. 


Number  of  Pairs. 

Outside  Diameter, 
Inches. 

Weights  per  1000  Feet 
in  Pounds. 

i 

2 

i 

- 

I 

214 
302 

3 

- 

515 

4 

i 

V 

629 

5 

1 

747 

6 

1 

i 

877 

7 

i 

i 

912 

10 

] 

I 

1214 

12 

i 

I 

1375 

15 

1566 

18 

20 

\i 

V 

1758 
1940 

25 

i~^ 

5* 

3232 

30 

35 

if 

2748 
2985 

40 

i 

V 

3176 

45 

i 

3365 

50 

i 

3678 

55 

i 

I 

3867 

60 

i 

4055 

65 

i- 

f 

4241 

70 

2 

4430 

80 

2 

^ 

4804 

90 

2- 

b 

5180 

100 

2 

1   . 

5505 

Aerial  cables  are  supported  on  steel  rope  stretched  tightly  be- 
tween the  poles  or  other  supports.  This  is  necessary  on  account 
of  the  fact  that  the  cable  does  not  possess  the  requisite  strength 
to  support  its  own  weight.  For  the  heavier  cables  the  messenger 
wire,  as  the  supporting  strand  is  called,  is  usually  composed  of 
seven  No.  8  steel  wires  twisted  together  into  a  rope.  Table  XVI. 
gives  the  common  sizes  of  messenger  wire,  together  with  their 
weights  and  breaking  strengths : 

A  special  strand  may  be  procured,  the  various  sizes  of  which 
have  about  double  the  breaking  strength  given  for  the  cor- 
responding sizes  in  this  table. 

Table  XVI.  is  useful  in  determining  the  size  of  cable  that  any 
messenger  wire  can  safely  carry  for  any  given  length  of  span. 
By  referring  to  the  table  giving  the  weights  per  1000  feet  of  cable 
and,  knowing  the  length  of  span,  the  size  of  messenger  wire  is 
readily  determined. 


OVERHEAD   CABLE   CONSTRUCTION. 


395 


TABLE  XVI. 
MESSENGER  AND  GUY  WIRE. 


7  Wires. 
No. 

Approximate     \ 
Diam.  in  Inches. 

Weight  per 
loo  Feet. 

Tensile  Strength 
in  Pounds. 

8 

\ 

52 

8320 

9 

10 

t 

42 
36 

6720 
5720 

ii 

29 

4640 

12 

TS 

21 

3360 

13 

3¥ 

16 

2560 

14 

ir 

12 

1920 

15 
16 

A 

10 

8 

1600 
1280 

17 

T3* 

6 

960 

18 
19 

1 

3ft 

688 
528 

20 

t 

2T% 

384 

21 

A 

2 

320 

TABLE  XVII. 

SUPPORTING  CAPACITY  OF  GALVANIZED  STEEL  STRANDS. 


5 

Spans  in  Feet. 

7  Wires. 

i2£ 

No. 

u~ 

100 

no 

120 

125 

130 

140 

150 

175 

200 

^ 

WEIGHTS  IN  POUNDS  OF  IOOO  FEET  OF  CABLE. 

8 

i 

28l8 

2516 

2263 

2152 

2050 

1867  !  1709 

I39i 

H54 

9 

if 

2520 

2247 

2O2O 

1920 

1827 

1663 

1520 

1234 

1130 

10 

TV 

2030 

1812 

1630 

1550 

1476 

1344 

1230 

1001 

9OO 

ii 

f 

1580 

1409 

1266 

I2O4 

1146 

1043 

953 

774 

640 

12 

A 

1  1  10 

SQQ 

890 

846 

805 

733 

670 

544 

450 

13 

A 

860 

765 

680 

652 

620 

563 

513 

414 

340 

15 

i 

585 

521 

468 

445 

423 

3«5 

352 

280 

235 

16 

A 

433- 

385 

346 

329 

313 

284 

260 

210 

172 

17 

337 

300 

270 

257 

245 

223 

204 

I65 

137 

The  messenger  wire  may  be  supported  in  several  ways,  one  of 
which  is  to  bolt  a  piece  of  angle  iron  to  the  pole  and  suspend  one 
messenger  wire  from  each  of  its  ends.  The  wire  may  be  sus- 
pended below  the  angle-iron  cross-arm,  or  it  may  rest  in  a  slot  in 


396 


AMERICAN   TELEPHONE   PRACTICE. 


it.     There  are  also  several  good  forms  of  supports  on  the  market, 
one  of  which  is  shown   in  Fig.  311. 

The  cable  is  supported  from  the   messenger  wire    in    several 
different  ways.      Rubber-covered   cable  is   frequently  suspended 


Fig.  311. — Messenger  Wire  Clamp. 

by  binding  it  to  the  messenger  wire  by  strong  tarred  marline. 
The  marline  is  wrapped  around  both  cable  and  messenger,  usu- 
ally in  two  directions,  to  give  greater  security.  The  method  now 
most  extensively  used  in  supporting  lead-covered  cables  is  by 
means  of  metallic  clips  or  hangers,  adapted  to  tightly  girdle  the 
cable  sheath  and  provided  with  a  hook  to  slip  over  the  support^ 


Figs.  312  and  313. — Cable  Hangers. 

ing  wire.  There  are  several  good  hangers,  two  styles  of  which 
are  shown  in  Figs.  312  and  313.  In  attaching  the  one  shown 
in  Fig.  312  the  metal  strip  which  passes  around  the  cable  is  first 
passed  for  a  distance  of  one  inch  through  the  slot  in  the  lower 
part  of  the  hook.  The  other  end  is  then  bent  around  the  cable 


OVERHEAD    CABLE   CONSTRUCTION.  397 

and  through  the  slotted  key.  The  key  should  then  be  turned 
to  the  left  until  tight  and  then  locked  by  driving  it  endwise  until 
the  ears  on  it  engage  in  the  star-shaped  hole.  The  flexible  strip 
in  this  hanger  is  made  of  zinc.  The  hanger  shown  in  Fig.  313  is 
of  malleable  iron  and  is  attached  by  bending  it  around  the  cable 
with  a  special  tool. 

It  is  a  good  plan  to  place  a  piece  of  sheet  lead  -fa  inch  thick, 
or  a  piece  of  leather  or  rubber  hose,  around  the  cable  at  the 
point  where  the  hanger  is  to  be  applied  ;  but  if  this  is  to  be  done 
the  additional  thickness  must  usually  be  allowed  for  in  ordering 
the  hangers. 

It  is  well,  in  ordering  cables,  to  specify  that  it  shall  be  placed 
upon  the  reels  in  such  manner  that  both  its  ends  are  accessible 
without  unreeling  it.  Where  this  is  done  it  is  an  easy  matter  to 


Fig.  314. — Running  up  Cable. 

make  tests  for  continuity  of  the  conductors  and  for  insula- 
tion resistance  and  capacity  before  the  cable  is  unreeled,  and 
thus  any  defects  which  may  exist  will  be  known  to  be  the  fault 
of  the  manufacturer  or  the  transportation  company.  When  the 
cable  arrives  both  of  its  ends  will  be  sealed  to  prevent  the  entrance 
of  moisture,  and,  after  testing,  the  ends  should  be  carefully  fe- 
sealed  in  a  manner  which  will  be  described  later.  The  ordinary 
method  of  hanging  cables  is  shown  in  Fig.  314,  which  is  taken 
from  Roebling's  "  Handbook  on  Telephone  Cables,"  as  are  several 
of  the  succeeding  cuts  illustrating  the  method  of  splicing.  The 
end  of  the  supporting  strand,  after  passing  over  the  last  clamp 
or  cable  cross-arm,  D,  is  firmly  secured  to  a  guy-stub  driven  in  the 
ground  at  A.  The  reel  on  which  the  cable  is  coiled  is  placed  in 
line  with  the  messenger  wire,  and  a  few  feet  beyond  the  stake,  as 


398  AMERICAN   TELEPHONE  PRACTICE. 

shown.  One  or  more  grooved  pulleys,  C  C,  mounted  as  shown, 
are  placed  between  the  reel  and  stake  in  such  manner  as  to  sup- 
port the  cable  as  it  is  paid  out.  A  stout  rope,  or  better  a  small 
wire  cable,  is  previously  hung  on  pulleys  or  hooks  below  the  cross- 
arms  of  the  entire  stretch  over  which  the  cable  is  to  be  drawn. 
One  end  of  this  is  attached  to  the  end  of  the  cable,  while  the 
distant  end  is  attached  to  a  capstan  or  other  form  of  windlass. 
As  the  cable  passes  over  the  rollers,  C  C,  the  hangers  are  at- 
tached and  are  placed  one  by  one  upon  the  inclined  messenger 
wire  as  they  reach  the  point,  B.  As  the  cable  progresses  line- 
men stationed  on  each  pole  lift  the  hangers  over  the  messenger 
wire  clamp  or  cross-arm  as  they  pass.  In  this  way  the  entire 
length  of  cable  is  drawn  up  to  and  along  the  stretch  without 
subjecting  any  portion  of  it  to  an  undue  strain.  The  hangers 
are  usually  attached  at  distances  of  from  twenty-four  to  thirty 
inches,  according  to  the  size  of  the  cable.  The  work  is  somewhat 
expedited  if,  during  the  drawing  up  of  the  cable,  only  about  every 
fifth  hanger  is  hooked  over  the  messenger  wire.  This  reduces 
the  labor  of  the  linemen  in  lifting  the  hangers  over  the  support. 
When,  however,  the  forward  end  of  the  cable  reaches  the  begin- 
ning of  the  last  span,  the  signal  should  be  given  to  all  linemen 
stationed  on  the  poles  to  hook  on  all  of  the  hangers  as  they  pass, 
and  in  this  way  all  of  the  hangers  will  be  secured  in  place  through- 
out the  entire  stretch  without  going  out  over  the  line  afterwards. 
This  method  is  subjected  to  one  disadvantage  in  that  the  slid- 
ing of  the  hanger  hooks  along  the  messenger  wire  tends  to  loosen 
them  on  the  cable,  sometimes  to  such  an  extent  that  several  of 
them  become  bunched  at  one  point  on  the  cable.  A  method  for 
overcoming  this  disadvantage,  and  also  for  expediting  the  work, 
has  been  devised  by  Mr.  F.  S.  Viele  of  the  Standard  Under- 
ground Cable  Company.  In  this,  carriers,  each  consisting  of  a 
small  grooved  roller  with  a  hook  below  it  for  engaging  the  cable, 
are  placed  upon  the  messenger  wire,  and  serve  to  support  the 
cable  at  frequent  intervals  instead  of  the  hangers  during  the  proc- 
ess of  stringing.  At  each  cross-arm  a  small  switch  or  side  track  is 
placed  upon  the  messenger  wire,  which  serves  to  displace  the  car- 
rier rollers  far  enough  to  clear  the  messenger  wire,  and  then  to 
guide  them  down  under  the  cross-arm  and  again  up  on  the  messen- 
ger wire.  These  side  tracks  are  about  three  feet  long  and  may 
be  readily  attached  or  detached  from  the  messenger  wire.  When 
the  forward  end  of  the  cable  reaches  the  beginning  of  the  last 
span  of  the  stretch  a  man  is  sent  up  each  pole  to  place  the  hang- 
ers on  the  messenger  wire  and  remove  the  carriers  as  they  pass, 


«    UNIVERSITY 
\0> 

OVERHEAD    CABLE    CONSTRUCTION.  399 

thus  leaving  the  entire  cable  permanently  suspended  when  the 
forward  end  reaches  its  destination. 

It  is  always  well  to  leave  sufficient  slack  in  aerial  cables  at  fre- 
quent intervals  to  allow  for  subsequent  splicing  in  case  repairs  are 
needed.  This  slack,  moreover,  frequently  saves  a  cable  from  se- 
rious injury  when  the  pole  line  is  subjected  to  some  severe  strain 
which  the  cable,  if  unable  to  give,  would  not  be  able  to  bear.  At- 
tention to  this  point  will  often  prevent  the  necessity  of  splicing 
in  a  new  piece  in  the  middle  of  a  cable,  due  to  insufficient  length 
for  making  an  ordinary  splice. 

Where  it  becomes  necessary  to  splice  a  cable  the  greatest  care 
should  be  taken  that  no  moisture  be  allowed  to  enter  while  the 
splice  is  being  made,  and  that  the  splice  shall  be  so  thoroughly 
sealed  at  the  end  of  the  operation  that  there  will  be  no  possibility 
of  the  subsequent  entrance  of  moisture.  A  suitable  staging 
should  be  erected  on  the  pole  where  the  splice  is  to  be  made,  if 


Fig.  315.— Cable  Prepared  for  Splicing. 

it  is  possible  to  bring  the  splice  within  reach  of  the  pole.  This 
can  always  be  provided  for  in  new  cable,  but  sometimes  in  repair- 
ing a  leak  it  is  necessary  to  make  these  splices  from  a  car  sus- 
pended from  the  messenger  wire.  When  all  is  ready  the  lead 
sheath  of  each  end  of  the  cable  to  be  spliced  should  be  cut  away 
for  a  distance  of  about  twelve  inches,  the  ends  of  the  cable  hav- 
ing previously  been  sawed  off  square.  Boiling  paraffin,  heated 
in  a  large  pan  on  a  portable  furnace,  should  then  be  ladled  over 
the  ends  of  the  wire  to  prevent,  as  far  as  possible,  any  moisture 
from  the  atmosphere  from  entering  the  cable  and  also  to  prevent 
the  untwisting  of  the  paper  insulation,  which,  in  a  dry-core  cable, 
often  gives  considerable  trouble  during  the  operation  of  splicing. 
A  lead  sleeve,  consisting  of  a  lead  pipe  about  two  feet  long,  and 
of  a  slightly  greater  internal  diameter  than  the  external  diameter 
of  the  cable  sheath,  should  then  be  slipped  over  one  end  of  the 
cable  and  back  several  feet  out  of  the  way.  A  paper  sleeve 
should  then  be  slipped  over  each  wire  of  each  pair.  The  pairs  in 
paper-covered  cables  are  usually  colored  red  and  white,  and  as  a 
matter  of  convenience  a  paper  sleeve  should  be  slipped  over  all 
of  the  red  wires  on  one  end  of  the  cable  and  one  of  the  white 
wires  on  the  other.  This  brings  the  cable  ends  into  the  condi- 


AMERICAN    TELEPHONE  PRACTICE. 

tion  shown  in  Fig.  315.  The  corresponding  wires  of  each  pair 
are  then  skinned  for  a  short  distance  and  twisted  together,  as 
shown  in  Fig.  316,  and  after  a  number  of  pairs  are  so  joined  all 


Figs.  316  and  317. — Splicing  a  Pair. 

joints  should  be  carefully  soldered,  particular  pains  being  taken 
to  use  no  acid  flux.  Tubular  solder  provided  with  a  rosin  flux 
inside  is  convenient  for  this,  or  the  grease  from  a  tallow  candle  is 
perhaps  better  yet.  With  a  rather  large  soldering  iron  these 
joints  may  be  soldered  almost  as  fast  as  the  iron  can  be  touched 
to  the  wire.  After  soldering,  the  twists  should  be  bent  down  as 
shown  in  Fig.  317,  after  which  the  paper  sleeves  are  slipped  over 
the  bare  portion  of  the  wires,  leaving  the  completed  splice,  as 
shown  in  Fig.  318.  The  joints  in  each  pair  should  not  lie  opposite 


Fig.  318. — Finished  Splice  on  Pair. 

each  other,  and  within  the  space  allowed  between  the  ends  of  the 
cable  sheaths  all  joints  should  be  staggered  as  much  as  possible 
so  as  to  prevent  the  formation  of  a  large  bunch  at  any  one  place. 
Another  point  to  be  remembered  is  to  guard  against  joining  any 
good  wires  in  one  cable  to  wires  known  to  be  bad  in  the  other. 
Before  making  these  splices  all  wires  should  be  tested  out  and  the 
bad  ones  tagged.  Obviously,  if  a  good  wire  in  one  length  of  the 
cable  is  attached  to  a  bad  one  in  another,  that  wire  is  unavilable 
for  use  in  either  cable.  After  all  of  the  wires  are  spliced  and 


Fig.  319. — Boiling  Out. 

covered  by  paper  sleeves  the  cable  should  be  carefully  "  boiled 
out,"  this  process  being  shown  in  Fig.  319  and  consisting  in 
ladling  boiling  paraffin  over  the  joint  until  all  traces  of  air  bub- 
bles in  the  hot  paraffin  disappear.  This  portion  of  the  work 


OVERHEAD   CABLE    CONSTRUCTION. 


401 


should  never  be  slighted,  as  it  is  one  of  the  most  important  in  the 
entire  operation. 

After  the  "  boiling   out"  process  is  completed  a  plain  strip  of 


Fig.  320. — Finished  Cable  Splice. 

white  cotton  should  be  wrapped  over  the  splicing,  after  which  the 
joint  should  again  be  boiled  out.     The  joint  is  now  ready  for  the 


Fig.  321. — Moon  Cable  Head. 

services  of  a  plumber,  and  upon  his  work  much  depends.  The 
section  of  pipe  should  be  slipped  over  the  splice  before  it  has 
had  time  to  cool,  and  the  sleeve  thoroughly  wiped  to  the  cable 


402 


AMERICAN    TELEPHONE   PRACTICE. 


sheath  at  each  end  by  the  ordinary  method  used  in  joining  lead 
pipes.  The  finished  joint  presents  the  appearance  shown  in  Fig. 
320,  and  when  such  a  joint  is  properly  made  that  portion  of  the 
cable  should  be  practically  as  good  as  any  other  portion. 

Whenever  it  is  necessary  to  leave  a  cable  end  exposed  all  moisture 
should  be  expelled  by  boiling  out,  after  which  the  end  of  the 
sheath  should  be  sealed  by  a  wiped  solder  joint  with  as  much  care 
as  if  it  were  to  be  a  permanent  affair.  If  it  is  suspected  that 
moisture  has  entered  the  end  of  a  cable  a  short  length  of  it  should 
be  cut  off  and  dipped  into  boiling  paraffin,  when  the  presence  of 
moisture  will  be  indicated  by  the  rising  of  bubbles  in  the  hot 


Fig.  322. — Fused  Terminal  Block  for  Moon  Head. 

fluid.  If  there  is  room  to  spare  the  cable  should  be  cut  back,  a 
short  length  at  a  time,  until  it  gives  evidence  of  being  dry,  but 
if  this  cannot  be  done  the  sheath  should  be  heated  with  a  torch, 
beginning  at  a  point  several  feet  from  the  end,  and  working  gradu- 
ally toward  the  end,  so  as  to  expel  the  moisture.  After  this  the 
ends  should  be  thoroughly  boiled  out  and  a  splice  made  as 
already  described,  or,  if  this  is  not  to  be  done,  the  end  should 
be  sealed. 

Where  a  cable  terminates  means  must  be  provided  for  distribut- 
ing its  various  wires  and  connecting  them  to  the  wires  forming  parts 
of  the  same  circuits.  For  this  purpose  what  are  termed  cable 
terminals  or  cable  heads  are  used,  several  forms  of  which  are  on 
the  market.  These  usually  consist  of  iron  boxes,  inside  of  which 
are  arranged  terminals  for  the  wires  in  the  cable.  The  lower  parts 
of  these  boxes  are  usually  provided  with  a  brass  tube  or  sleeve 
adapted  to  fit  over  the  cable  sheath,  after  which  it  is  secured 
thereto  by  a  plumber's  wiped  joint,  thus  hermetically  sealing  both 


OVERHEAD   CABLE   CONSTRUCTION. 


403 


cable  head  and  sheath  at  that  point.  The  wires  of  the  cable  are 
fanned  out  to  the  terminals  within  the  box,  which  terminals  are 
usually  connected  through  water-tight  insulating  bushings  with 
their  terminals  outside  of  the  boxes.  After  the  various  connec- 
tions are  made  within  the  box  a  cast-iron  cover  is  screwed  in  place, 
the  joints  being  hermetically  sealed  by  a  rubber  gasket.  On  the 
outside  of  the  boxes  are  usually  provided  lightning  airresters  for 


Figs.  323  and  324. — Cook  Pole  Top  Terminal. 

each  line,  the  circuit  being  completed  from  the  inner  connectors 
through  these  arresters  to  the  outer  wires.  One  of  these,  known 
as  the  Moon  cable  head,  is  clearly  shown  in  Fig.  321,  in  which 
several  wires  are  shown  extending  from  the  tube  below  the  box  to 
one  of  the  terminals  within.  In  Fig.  322  is  shown  a  fused  termi- 
nal for  use  with  the  Moon  head.  The  bushing  which  serves  to 
lead  the  terminal  pin  through  the  iron  casing  also  carries  a  brass 
lug  between  which  and  the  pin  is  placed  the  fuse,  mounted  on  a 
hard-rubber  block  as  clearly  shown.  Another  form  of  cable  head 


4°4 


AMERICAN   TELEPHONE   PRACTICE, 


which  is  becoming  very  popular  is  known  as  the  Cook  pole-top 
terminal,  views  of  which  are  shown  in  Figs.  323  and  324. 
In  this,  which  is  the  invention  of  Mr.  Frank  B.  Cook  of  the  Ster- 


Fig.  325.— Pole  Equipped  with  Cook  Terminal. 

ling  Electric  Company,  the  cable  is  led  up  within  the  cast-iron 
box,  forming  the  framework  for  an  entire  terminal,  the  various 
wires  being  fanned  out  to  conductors  arranged  in  circular  rows 
around  the  inside  of  the  box.  Connection  is  made  through  suit- 
able air-tight  plugs  with  outside  circuits,  the  line  wires  being 
fused  at  the  insulator,  as  shown  at  the  left-hand  lower  portion 


OVERHEAD    CABLE    CONSTRUCTION'.  405 

of  Fig.  324.  Before  screwing  on  the  cover  several  lumps  of  un- 
slacked  lime  are  placed  within  the  box,  after  which  the  cover  is 
screwed  on,  being  hermetically  sealed  by  rubber  gasket  as  in 
the  other  form  described.  This  lime  absorbs  any  slight  mois- 
ture which  may  be  in  the  box  at  the  time,  and  the  fact  that  it  has 
in  many  cases  remained  unslaoked  for  years  proves  conclusively 
that  these  terminals  may  be  made  perfectly  moisture-proof.  Af- 
ter all  connections  are  made  a  sheet-iron  cover  is  placed  over  the 
entire  terminal. 

This  terminal,  as  its  name  implies,  is  placed  at  the  top  of  the 
pole  in  a  manner  shown  in  Fig.  325. 

A  much  less  expensive  method  of  terminating  cables  than  any 
so  far  described  consists  in  the  use  of  what  are  termed  pot-heads, 
and  while  these  present  a  somewhat  homely  appearance  they  are 
very  effective,  and  are  used  to  a  large  extent  by  many  of  the 
Bell  and  other  companies.  There  is  no  doubt  but  that  a  pot- 
head  terminal,  properly  constructed,  forms  as  reliable  and  service- 
able a  terminal  as  any,  it  having  the  additional  advantage  of  being 
far  cheaper  than  any  of  the  others.  The  directions  for  making 
these,  together  with  the  description  of  all  material,  are  given 
in  the  following  specifications,  which  are  those  of  one  of  the 
leading  Bell  companies. 

POT-HEAD   TERMINALS. 

Materials. 

Lead  Sleeves — of  unalloyed  lead  \  inch  thick  of  the  following 
dimensions : 

For  loo-pair  cable  ;  length,  24  inches,  inside  diameter,  3  inches. 

«         HQ       "  "  "  2O        "  "  "  2\      " 

«          2£         «  «  «  20        «  u  «  2 

Drift  out  the  sleeve  for  one-half  its  length  until  its  diameter  is 
increased  J  of  an  inch. 

Okonite  Wire,  twisted  pair,  red  and  black  No.  20  B.  &  S.  gauge, 
£32-inch  insulation,  without  braid  or  outside  covering. 

Okonite  Tape,  f  inch  wide. 

Paper  Sleeves,  boiled  in  paraffin  just  before  using. 

Brass  Tubing—  thin,  £  inch  in  diameter,  length  2\  inches  less 
than  that  of  lead  sleeves. 

Heavy  Cotton  Twine,  or  wicking. 

Wiping  Solder,  containing  40  per  cent.  tin. 

Splicing  Compound,  as  furnished  for  the  purpose  by  the  com- 
pany. Do  not  mix  the  compound  with  other  materials. 


406 


AMERICAN    TELEPHONE   PRACTICE. 


Directions. 

Remove  the  cable  sheath  for  fifteen  inches,  slip  the  lead 
sleeve  over  the  cable,  splice  the  cable  wires  to  okonite  wire  in 
the  usual  manner,  join  the  colored  wire  of  the  cable  to  the  red 


Fig.  326. — Cable  Terminal  Box  with  Balcony. 


okonite  of  each  pair,  cover  each  splice  with  a  sleeve,  and  keep  the 
splices  within  a  limit  of  thirteen  inches  from  the  end  of  the  cable 
sheath.  Remove  all  pieces  of  paper  or  other  debris,  bind  the 
cable  wires  as  they  leave  the  sheath  tightly  with  several  layers  of 
twin6  to  prevent  the  compound  entering  the  cable,  tape  all  the 
okonite  wires  together  for  three  inches  in  such  a  manner  that  one- 
half  inch  of  the  taped  wire  will  be  below  the  surface  of  the  com- 


OVERHEAD    CABLE   CONSTRUCTION.  4°  7 

pound.  Open  up  the  spliced  wires  as  much  as  possible  to  allow 
free  spaces  between,  bind  the  brass  tube  with  twine  alongside 
the  wires  with  the  lower  end  even  with  the  end  of  the  sheath  of 
the  cable,  binding  no  higher  than  the  wire  splices;  draw  up  the 
lead  sleeve  until  its  lower  end  laps  over  the  cable  sheath 
1 1  inch,  and  wipe  it  to  the  sheath.  Secure  the  whole  in  an  up- 


Fig.  327. — Interior  Cable  Heads. 

right  position,  warm  the  lead  sleeve  until  it  can  barely  be  touched 
with  the  hand,  place  a  funnel  in  the  brass  tube,  and  slowly 
pour  in  the  sealing  mixture,  previously  heated  to  350°  F.r 
until  it  fills  the  sleeve  to  within  one-half  inch  of  the  top, 
remove  the  funnel  and  allow  the  compound  to  settle  and  cool. 
Test  the  compound  just  before  using  by  putting  in  a  short  piece 
of  okonite  wire  for  two  minutes.  If  the  okonite  is  not  softened 
so  as  to  readily  come  off  the  wire  the  compound  is  not  too  hot. 
Protect  the  open  end  and  the  wires  leading  therefrom  against  the 


40  8  AMERICAN    TELEPHONE   PRACTICE. 

weather.  On  the  next  day  fill  with  hot  compound  to  make  up 
for  the  settlement.  Three  days  later  do  the  same  if  necessary. 
After  this,  and  when  thoroughly  cold,  dress  the  top  of  the  lead 
sleeve  into  contact  with  the  okonite  tape  wrapping,  which  at  this 
point  should  consist  of  at  least  four  layers.  Place  a  cross  on  the 
outside  of  the  lead  sleeve  at  a  point  opposite  the  upper  end  of 
the  brass  tube.  Caution  :  Do  not  boil  out  the  cable  end  with 
paraffin.  If  dampness  enters,  the  cable  should  not  be  used  until 
this  defect  is  remedied  or  the  part  cut  away.  Under  no  circum- 
stances must  paraffin  be  used  on  okonite  ends. 

The  okonite  paired  wires  shall  project  above  the  end  of  the 
flexible  end  for  a  distance  of  : 

For  100  pairs  cables  3  feet. 

"       50        "  "        2      " 


Cable  heads,  whether  of  the  ordinary  iron  or  the  pot-head  type, 
when  placed  on  poles  are  usually  inclosed  in  a  wooden  box  of 
suitable  dimensions  for  allowing  connections  with  the  external 
circuits.  These  boxes  should  be  provided  with  hinged  doors  and 
made  as  nearly  water-tight  as  possible.  Directly  below  these  boxes 
should  be  erected  a  balcony  upon  which  the  workmen  may  stand 
when  making  connections  for  testing  the  various  lines.  Such 
construction  is  well  shown  in  Fig.  326. 

In  Fig.  327  is  shown  a  row  of  eight  cable  terminals  used  in 
terminating  as  many  one  hundred  pairs  of  cables  within  an  ex- 
change. These  particular  terminals  are  the  product  of  the  Ster- 
ling Electric  Company,  and  are  provided  with  combined  sneak 
current  and  carbon  arresters  for  each  line. 


CHAPTER  XXXII. 

( 

UNDERGROUND   CABLE   CONSTRUCTION. 

THE  tendency  at  present  is  to  place  all  telephone  wires  in 
cities  underground,  and  the  primary  requisite  for  this  construc- 
tion is  that  a  suitable  conduit  shall  be  provided  in  which  the  con- 
ductors may  be  laid.  It  is  usually  necessary  to  provide  conduits 
having  a  suitable  number  of  ducts  to  meet  the  requirements  for 
future  as  well  as  immediate  use,  and  much  judgment  should  be 
exercised  in  this  respect  in  planning  the  system.  Suitable 
openings  are  provided  for  the  conduits  at  frequent  intervals, 
these  being  in  the  form  of  man-holes,  from  which  sections  of  the 
cables  may  be  drawn  into  the  ducts  and  withdrawn  when  occasion 
requires  for  repairs.  The  principal  requirements  for  a  good  con- 
duit may  be  outlined  as  follows  : 

The  material  of  which  the  conduit  is  made  must  be  durable, 
and  this  implies  that  it  must  be  absolutely  proof  against  decay  or 
corrosion  due  to  moisture,  dry  rot,  gases,  or  the  liquids  present 
in  the  soil.  It  should,  moreover,  be  fire-proof  if  possible,  al- 
though this  is  a  minor  consideration. 

The  conduit  should  possess  both  tensile  shearing  and  crush- 
ing strength.  Severe  vertical  strains  are  frequently  imposed  upon 
subway  structures,  due  to  the  removal  of  the  support  from  be- 
neath them,  caused  by  excavations  in  the  streets  or  by  the 
settling  of  the  ground.  Side  strains  are  not  so  likely  to  occur, 
and  their  effects  are  usually  slight ;  therefore,  it  follows  that 
the  conduit  should  be,  if  possible,  strongest  in  a  vertical  direc- 
tion. If  the  stress  imposed  upon  the  structure  is  such  as  to  cause 
a  fracture  or  undue  settling,  the  alignment  of  the  ducts  is  thereby 
destroyed,  which  may  interfere  with  the  drawing  in  or  with- 
drawal of  cables.  Moreover,  the  grade  of  the  duct  is  destroyed, 
so  that  the  proper  drainage  cannot  be  effected.  The  ducts  be- 
tween man-holes  should  be  straight  if  possible,  and  where  curves 
are  necessary  they  should  be  very  gradual  and  present  no  sharp  cor- 
ners which  would  interfere  with  the  drawing  in  or  seriously  abrade 
the  cable  sheath.  Slight  turns  in  conduits  are  frequently  made 
by  joining  together  short  straight  sections,  but  where  the  nature 
of  the  conduit  used  permits  it,  it  is  better  to  form  all  bends  of 

409 


410  AMERICAN   TELEPHONE  PRACTICE. 

curved  sections.  It  is  desirable  that  the  structure  should  be  com- 
posed of  insulating  material  and  be  moisture-proof.  No  depend- 
ence, however,  for  insulation  of  the  conductors  themselves  must 
be  placed  on  the  conduits,  as  the  cables  must  in  all  cases  provide 
the  means  for  keeping  the  conductors  thoroughly  insulated  and 
free  from  moisture,  even  under  the  most  adverse  circumstances. 
Even  the  most  perfectly  constructed  conduits  cannot  be  kept 
dry,  on  account  of  the  sweating  of  their  interior  walls. 

It  is  very  essential  that  the  conduit  must  contain  no  chemical 
agents  capable  of  exerting  a  deleterious  effect  on  the  cable 
sheath.  As  an  example  of  this  may  be  mentioned  certain  forms 
of  wooden  conduits,  which  in  the  process  of  decay  liberate  acetic 
acid,  which  in  a  short  time  totally  destroys  the  cable  sheath, 
changing  it  to  lead  acetate.  This  difficulty  has  been  experienced 
with  some  forms  of  creosoted  wood  conduit,  but  in  the  later 
products  in  this  line  this  difficulty  is  said  to  have  been  com- 
pletely removed  by  the  use  of  a  better  grade  of  creosote  oil  and 
improved  methods. 

Economy  of  space  is  often  an  important  item  in  the  selection 
of  conduit  to  be  used,  and  under  crowded  conditions  that  con- 
duit which  will  place  a  given  number  of  ducts  within  the  smallest 
space  is  the  most  desirable,  other  things  being  equal. 

The  earliest  form  of  conduit  used  in  this  country  was  the 
open-box  conduit,  which  consisted  merely  in  a  trough  made  of 
inch-and-a-half  or  two-inch  lumber  and  of  sufficient  size  to 
accommodate  enough  cables  to  meet  the  existing  demands, 
as  well  as  the  future  growth  of  the  system.  These  troughs 
were  laid  in  a  trench,  the  bottom  of  which  was  properly 
graded,  the  sections  of  the  trough  being  about  fifteen  feet 
in  length  and  butt-jointed — that  is,  laid  together  end  to  end. 
The  joints  were  held  in  line  by  boards  nailed  on  the  outside  and 
overlapping  each  end  about  a  foot.  The  cable  was  laid  in  these 
troughs  by  driving  the  reel  containing  it  slowly  alongside  of  the 
trench,  the  cable  being  carefully  laid  as  it  was  unwound  from  the 
reel.  After  all  the  cables  were  in  place  the  trough  was  filled 
with  hot  pitch,  when  the  cover  was  nailed  in  position  and  the 
trench  refilled.  This  is  probably  the  simplest  form  of  under- 
ground cable  construction,  with  the  exception  of  a  method  some- 
times practiced  in  Europe,  of  laying  the  cable  directly  in  the 
ground  without  any  conduit  whatever. 

The  cheapest  and  simplest  form  of  conduit  which  permits  the 
drawing  in  or  withdrawal  of  the  cables  is  that  composed  of  creo- 
soted wood  tubes,  or  "  pump  logs,"  as  they  are  commonly 


UNDERGROUND    CABLE   CONSTRUCTION. 


411 


and  appropriately  termed.  These  are  usually  made  in  eight- 
foot  lengths,  having  a  square  external  section  4^  x  4^  inches,  with 
a  3-inch  bore.  A  tenon  joint  one-and-one-half  inch  long  is  used 
for  securing  proper  alignment  of  the  joint.  Several  views  of 
this  tube  are  shown  in  Fig.  328.  The  wood  is  treated  with  creo- 
sote or  dead  oil  of  coal  tar  in  the  following  manner:  The  lum- 
ber is  laid  on  cars  and  run  into  a  large  steel  cylinder  six  feet  in 


Fig.  328.—  Creosotecl  Wood  Conduit. 

diameter,  which  is  closed  by  a  heavy  iron  door.  It  is  first  sub- 
jected to  live  steam  at  a  temperature  of  250°  F.  until  the  timber 
is  heated  through  and  through,  the  purpose  of  this  being  to  co- 
agulate the  albumen  in  the  sap.  A  vacuum  pump  is  next  applied 
to  the  tank,  exhausting  all  air  and  steam,  the  pump  maintaining 
a  vacuum  of  about  twenty-six  inches.  This  evaporates  practi- 
cally all  of  the  sap  and  water  from  the  wood,  thus  seasoning  the 
timber.  The  next  step  in  the  process  is  to  pump  creosote  oil  pre- 
viously heated  to  a  temperature  of  100°  to  125°  F.  into  the  tank 
until  it  is  full.  This  is  then  placed  under  a  pressure  of  about 
eighty  pounds  per  square  inch  and  the  amount  of  creosote  which 
is  forced  in  after  the  filling  of  the  tank  is  carefully  measured, 
this  being  the  amount  that  is  taken  up  by  the  pores  of  the 
wood.  Specifications  for  the  treatment  require  that  from  eight 
to  twenty  pounds  of  the  oil  shall  be  absorbed  by  each  cubic  foot 
of  timber.  Twelve  or  fifteen  pounds  is  the  average  amount  re- 
quired for  electrical  purposes.  As  has  been  stated  before,  much 
trouble  has  existed  owing  to  the  liberation  of  acetic  acid  from 
conduit  treated  with  creosote.  It  is  claimed,  however,  that  by 
using  a  proper  quality  of  creosote  oil,  and  by  using  the  method 
of  impregnation  just  described,  that  this  trouble  has  been  entirely 
eliminated.  The  life  of  creosoted  wood  conduit  is,  to  say  the 
least,  problematical,  but  there  seems  to  be  good  reason  to  believe 
that  when  properly  treated  and  laid  it  will  last  an  ordinary  life- 
time, if  not  longer.  In  laying  this  conduit  the  trench  is  dug  to  a 


412  AMERICAN   TELEPHONE   PRACTICE. 

sufficient  depth,  and  after  its  bottom  is  properly  graded  so  as  to 
have  a  gradual  slope  either  from  an  intermediate  point  toward 
both  man-holes  or  an  uninterrupted  slope  from  one  man-hole  to 
the  other,  a  creosoted  wood  plank  two  inches  thick  is  laid  as  a 
foundation.  The  ducts  are  then  laid  on  this  plank  side  by  side 
and  in  as  many  different  layers  as  are  necessary  to  give  the  re- 
quired number.  They  should  be  so  laid  that  the  separate  ducts 
break  joints  in  order  to  give  strength  to  the  entire  struc- 
ture. Over  the  upper  layer  is  then  laid  another  creosoted  wood 
plank  two  inches  thick,  after  which  the  trench  is  filled  in  with 
earth.  The  great  point  in  favor  of  this  conduit  is  its  cheapness, 
this  being  greatly  enhanced  by  the  fact  that  no  concrete  is  em- 
ployed for  a  foundation. 

Conduits  of  clay  or  terra-cotta,  burned  hard  and  with  vitrified 
surface,  are  being  extensively  used  and  are  giving  unqualified 
satisfaction.  These  are  made  up  in  a  number  of  forms  which 
may  be  divided  into  two  classes,  namely,  multiple  duct  and  single 
duct.  The  multiple-duct  conduit  is  made  up  in  a  variety  of 
ways,  some  of  which  are  shown  in  cross-section  in  Figs.  329,  330, 


Fig.  329. — Multiple  Duct  Conduit. 

and  in  the  upper  portion  of  331.  The  sections  of  conduit  shown 
in  Fig.  329  usually  have  a  cross-section  lox  10  inches  and  a  length 
of  three  feet.  Similar  tiles  are  frequently  used  having  six  or  eight 
ducts,  each  about  3^-  x  3^  inches  square,  the  tiles  varying  from 
three  to  six  feet  in  length.  Frequently  the  ducts  are  made  large 
enough  to  accommodate  several  cables,  but  this  has  a  decided 
disadvantage,  owing  to  the  fact  that  much  trouble  is  experienced 
in  withdrawing  cables  under  these  conditions.  The  successive 
lengths  of  these  tiles  are  joined  by  wrapping  them  with  burlap 
previously  dipped  in  asphalt.  This  makes  the  joint  tight,  main- 
tains its  alignment,  and  prevents  entrance  of  dirt  during  subse- 
quent operations. 

The  conduit  shown  in  Fig.  330  is  valuable  only  where  multi- 
ples of  four  ducts  are  required.  Each  tile  is  essentially  a  trough, 
open  on  top  and  having  three  intermediate  partitions,  thus  form- 
ing four  ducts,  each  of  which  is  3f  inches  wide  by  four  inches 
high,  the  walls  being  one  inch  thick.  These  are  made  in  two-foot 
lengths,  and  are  laid  in  concrete  as  shown,  one  section  being  laid 
directly  on  top  of  another  until  the  required  number  of  layers  are 


UNDERGROUND   CABLE    CONSTRUCTION. 


413 


formed.  The  top  of  the  upper  section  is  composed  of  a  sheet  of 
mild  steel  of  No.  22  B.  W.  G.,  having  its  edges  bent  down  so  as 
to  lap  over  the  sides  of  the  tile. 

The  tile  shown  in  the  upper  portion  of  Fig.  331  was  designed 
by  Mr.  H.  W.  Johnston  of  St.  Louis  for  the  purpose  of  distribut- 


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//I  ---'  ^^x^^r-its?  r!~- 

Fig.  330. — Four-Duct  Terra-Cotta  Tile. 

ing  the  wires  from  the  main  cable,  running  to  a  certain  section,  to 
the  various  lateral  ducts.  The  lower  central  duct  is  for  the  main 
cable  running  to  the  man-hole  at  the  center  of  distribution. 
From  this  it  branches  out  through  a  junction  box  to  the  distrib- 
uting wires,  the  two  four-inch  openings  on  either  side  being  for 


Fig.  331. — Johnston  Distributing  Duct. 

these  wires  or  cables,  which  are  led  out  through  holes  in  the  side 
wall  of  the  duct  to  the  lateral  ducts,  consisting  of  three-inch 
iron  pipe.  The  upper  central  duct  is  for  rodding  and  drawing  in 
of  additional  wires  in  the  side  ducts.  A  runner  is  drawn  through 
the  central  upper  duct  with  arms  projecting  over  the  side  ducts, 
and  by  means  of  these  projecting  arms  new  wires  may  be  drawn 


414  AMERICAN    TELEPHONE   PRACTICE. 

into  the  side  duct  without  in  any  way  disturbing  those  already  in 
place. 

The  single-duct  class  of  tiles  possesses  some  advantages  over 
the  multiple-duct  tiles,  chief  among  which  are  the  greater  flexibil- 
ity and  the  increased  ease  of  handling.  The  form  shown  in  Fig. 
332  has  come  into  very  wide  use  and  has  proven  its  adaptability 


Fig.  332. — Single-Duct  Conduit. 

to  meet  almost  any  conditions  that  may  arise.  These  tiles  are 
4|  inches  square  by  18  inches  long,  and  have  a  3J--inch  bore.  By 
it  curves  are  easily  made,  short  curved  lengths  being  provided,  or 
curves  of  long  radius  may  be  made  with  the  regular  tiles,  the 
lengths  being  so  short  as  to  form  a  practically  smooth  interior 
surface.  This  conduit  is  laid  in  much  the  same  way  as  ordinary 
brick,  and  in  order  to  insure  proper  alignment  a  mandrel  (Fig. 
333),  three  inches  in  diameter  and  about  thirty  inches  long,  is  laid  in 


Fig.  333. — Mandrel. 

the  duct  and  pulled  along  through  it  by  the  workmen  as  each  ad- 
ditional section  is  laid  on.  The  rear  end  of  this  mandrel  is  pro- 
vided with  a  rubber  basket  a  little  larger  than  the  diameter  of  the 
conduit,  which  effectually  smooths  the  inner  surface  and  pre- 
vents the  formation  of  lips  which  might  prove  injurious  to  the 
cable  sheaths  in  drawing  in.  On  the  front  end  of  the  mandrel  is 
provided  an  eye  which  may  be  engaged  by  a  hook  carried  by  the 
workmen  in  order  to  move  it  forward.  Fig.  334  is  a  photograph 
showing  a  48-duct  subway  in  process  of  construction.  In  laying 
vitrified  clay  tile  of  any  description  the  process  to  be  used  is  as 
follows  :  The  trench  is  dug  to  such  a  depth  as  to  allow  at  least 
two  feet  of  earth  above  the  top  of  the  entire  structure.  Some 


UNDERGROUND  CABLE    CONSTRUCTION. 


415 


specifications  call  for  as  great  depth  as  three  feet,  but  this  is  nec- 
essary only  where  there  is  a  probability  that  new  ducts  may  be 
added  to  the  conduit  in  the  future.  The  width  of  the  trench 
should  be  at  least  eight  inches  in  excess  of  the  actual  width  of 
the  number  of  ducts  which  are  to  be  laid  side  by  side.  In  the 
bottom  of  the  trench  is  laid  a  (concrete  foundation  to  a  depth  of 
from  three  to  six  inches— four  inches  is  under  ordinary  circum- 
stances sufficient.  The  tiles  are  then  laid  in  place  in  cement 
mortar,  and  as  each  layer  is  finished  the  sides  of  the  trench  should 


Fig.  334. — Forty-Eight  Duct  Subway,  Cleveland,  Ohio. 

be  filled  to  the  top  of  that  layer  with  the  same  concrete  as  that 
used  for  the  foundation.  The  space  between  the  tiles  in  a  layer 
and  between  the  layers  should  be  carefully  filled  with  good  cement 
mortar  mixed  thin  enough  to  readily  fill  the  interstices.  After 
the  required  number  of  layers  are  in  place  the  top  is  covered  with 
a  mass  of  concrete  not  less  than  four  inches  in  thickness. 

The  concrete  used  in  this  work  should  be  composed  of  one  part 
of  hydraulic  cement,  two  parts  of  clean  sharp  sand,  and  five  parts 
of  broken  stone,  screened  gravel,  or  broken  brick.  The  size  of  the 
broken  stone  or  brick  or  gravel  should  not  be  larger  than  one  inch 
in  any  dimension.  The  cement  and  sand  should  be  thoroughly 
mixed  while  dry,  and  then  enough  water  added  to  form  a  soft 


416  AMERICAN    TELEPHONE  PRACTICE. 

mortar,  after  which  the  broken  stone  should  be  thoroughly  mixed 
in. 

The  mortar  should  be  composed  of  one  part  of  hydraulic  ce- 
ment and  two  parts  of  clean  sharp  sand,  thoroughly  mixed  to- 
gether, and  then  with  water  as  before.  It  is  a  matter  of  greatest 
importance  that  the  ducts  should  not  be  moved  while  the  mor- 
tar or  concrete  is  setting. 

After  the  entire  subway  is  laid  from" one  man-hole  to  the  other  it 
is  advisable  to  draw  through  it  a  scraper,  thus  removing  all  projec- 
tions on  the  inside  walls.  The  ducts  may  then  be  washed  out  with 
a  hose,  thus  removing  all  grit  and  leaving  a  clean,  polished  tube. 

Another  style  of  conduit  is  the  cement  line  pipe,  which  has 
also  proven  itself  to  be  thoroughly  reliable  in  all  respects.  This, 
as  usually  constructed,  consists  of  a  wrought-iron  pipe  of  No. 
26  B.  W.  G.,  with  riveted  joints,  the  rivets  being  set  one  and 
one-half  inch  apart.  This  pipe  is  lined  with  Rosendale  cement, 
the  thickness  of  the  lining  being  five-eighths  of  an  inch,  and  the 
interior  of  the  lining  being  polished.  The  standard  size  of  this 
tube  is  in  eight-foot  lengths,  with  a  three-inch  bore.  It  is  pro- 
vided with  cast-iron  ball  and  socket  joints  at  the  ends  in  order  to 
insure  proper  alignment  and  to  provide  a  certain  amount  of  flex- 
ibility in  making  turns. 

This  conduit  is  laid  in  concrete  in  much  the  same  manner  as 
the  clay  pipe,  it  being  common  practice  to  separate  the  different 
pipes  in  the  layer  by  about  one-half  of  an  inch  and  the  various 
layers  themselves  by  about  one  inch. 

Another  form  of  conduit,  radically  different  from  all  those  so  far 
described,  is  the  Sewall  cement  arch  conduit,  which  is  a  recent  pro- 
duction, but  which  gives  great  promise  of  success.  The  cross- 
section  of  a  single  duct  is  shown  in  Fig.  335,  while  in  Fig.  336  is 


Figs-  335  and  336.—  Cement-Arch  Conduit. 


shown  in  perspective  the  method  of  joining  two  lengths. 
The  separate  ducts  are  formed  of  cement  molded  around  a  strip 
of  wire  gauze,  as  shown  in  Fig.  335.  The  arch  is  made  of  equal 
parts  of  Portland  cement  and  sand,  being  strengthened  by  the 
metal  gauze,  which  is  made  from  No.  19  B.  &  S.  gauge  iron 
wire,  woven  with  a  mesh  three-eighths  of  an  inch  square.  The 


UNDERGROUND   CABLE    CONSTRUCTION.  4*7 

inside  measurement  of  the  standard  size  is  three  inches  wide 
and  three  inches  from  the  top  of  the  arch  to  the  base.  In  lay- 
ing these  arches  the  trench  is  dug  in  the  usual  manner  and  a 
concrete  foundation  laid.  The  top  of  the  foundation  is  made 
smooth  to  grade,  being  troweled  to  a  polish.  The  arches  are  then 
wet  and  placed  on  the  floor  so  fbrmed  under  a  templet  which  gives 
them  accurate  alignment.  Joints  are  made  by  abutting  the  sec- 
tions end  to  end.  Over  the  joint  is  placed  a  wire  gauge  bridge,  as 
shown  in  Fig.  336,  the 'wire  being  lined  inside  with  cotton  cloth, 
to  which  is  cemented  a  similar  cloth  upon  the  outside.  As 
the  arches  are  accurately  molded,  and  as  the  floor  is  supposed 
to  be  perfectly  plain,  the  joints  so  formed  present  a  smooth 
interior.  As  soon  as  the  first  tier  of  arches  is  in  position  it  is 
immediately  covered  with  concrete,  which  is  then  smoothed  down 
to  form  a  second  floor,  upon  which  the  second  tier  of  arches  is 
laid.  After  the  required  number  of  ducts  are  in  place,  the  usual 


Fig.  337. — Twelve-Duct  Cement  Arch  Conduit. 

layer  of  concrete  is  placed  over  the  entire  structure.  In  Fig.  337 
is  shown  a  sectional  view  of  a  12-duct  conduit  of  this  type. 

Curved  sections  of  these  arches  are  made  for  making  bends, 
these  curves  usually  being  made  in  6-inch  lengths.  The  cross- 
section  of  the  duct  in  this  conduit  possesses  some  advantage  over 
the  round  duct.  If  foreign  matter  finds  its  way  into  the  duct 
after  the  cable  is  in  place,  or  if  a  piece  of  the  external  braiding- 
from  a  cable  sheath  is  torn  off,  it  will  not  be  so  likely  to  bind  the 
cable  in  drawing  it  out  as  in  the  circular  form  of  duct,  because 
the  foreign  matter  will  be  likely  to  sink  down  in  the  lower  cor- 
ners of  the  duct,  where  considerable  room  is  provided  for  it. 

In    laying    conduit    in  city  streets  numerous  obstructions  are 


418 


AMERICAN    TELEPHONE   PR  ACIDIC E. 


met,  and  must  be  overcome  in  the  manner  best  suited  to  the 
individual  case.  It  frequently  becomes  necessary  to  remove  the 
support  from  heavy  pipe  lines  for  a  considerable  distance,  as, 
for  instance,  when  such  a  pipe  line  lies  diagonally  across  the  trench. 
In  all  cases  suitable  supports  for  these  pipes  or  other  structures 
should  be  provided  until  such  time  as  the  trench  is  again  filled. 


Fig.  '338. — Avoiding  Obstacles. 


The  usual  means  adopted  is  to  place  a  beam  of  sufficient 
strength  across  the  top  of  the  trench  and  support  the  pipe 
therefrom  by  chains  or  heavy  rope.  It  is  frequently  necessary 
in  passing  an  obstruction  to  fan  out  the  pipes  in  one  layer  so 
that  they  occupy  the  same  level  as  those  of  another  layer.  Such 
a  construction,  and  also  a  rather  crooked  piece  of  conduit  work, 
is  shown  in  Fig.  338,  where,  on  account  of  obstructions  in 
the  street,  the  two  layers  of  two  pipes  were  formed  into 
one  layer  of  four  until  the  obstructions  were  passed.  This  par- 


UNDERGROUND   CABLE   CONSTRUCTION. 

ticular   obstruction    was  a  sub-cellar   extending    out    under   the 
street. 

The  man-holes  may  be  built  of  various  forms  and  dimensions 
to  meet  existing  requirements.  In  the  best  construction  the 
foundation  consists  of  a  layer  of  concrete  six  inches  deep,  the 
concrete  being  mixed  as  specified  for  the  laying  of  tiles,  with 
the  exception  that  the  crushed  stone  may  be  considerably  coarser. 
The  walls  of  the  man-hole  are  then  built  of  good  brick-work  of 
suitable  thickness  and  well  plastered  on  the  outside  with  cement 
mortar  in  order  to  exclude  as  much  dampness  as  possible.  For 
the  ordinary  man-hole  an  eight-inch  wall  is  sufficiently  thick, 
but  in  building  very  large  underground  vaults  it  sometimes  be- 
comes necessary  to  double  or  triple  this  thickness.  Where  these 
very  thick  walls  are  required  it  is  good  practice  to  allow  about 
one  inch  air  space  between  the  outer  course  of  brick  and  the 
inner  in  order  to  render  the  interior  as  dry  as  possible.  A 
common-sized  man-hole  is  five  by  five  by  five  feet,  and  smaller 
sizes  down  to  three  by  three  feet  with  five  feet  head  room  are 
also  common.  As  a  rule  a  man-hole  should  provide  at  least 
enough  room  for  two  men  to  work  in  conveniently.  Of  course, 
where  a  great  number  of  ducts  enter  a  man-hole  the  size  must 
be  increased  accordingly. 

After  the  conduits  are  laid  and  the  man-holes  finished  the 
next  step  is  the  drawing  in  of  the  cables.  In  order  to  accom- 
plish this  a  process  called  rodding  is  in  most  cases  first  neces- 
sary, in  order  that  a  rope  may  be  stretched  through  the  duct, 
which  is  afterwards  to  be  used  for  drawing  in  the  cable  itself. 
For  this  purpose  a  large  number  of  wooden  rods  about  three- 
fourths  of  an  inch  in  diameter  and  four  feet  long,  and  equipped 
with  screw  or  bayonet  joints  at  each  end,  so  that  they  may 
readily  join  together,  are  necessary.  A  man  stationed  in  one 
of  the  man-holes  inserts  one  rod  into  the  duct,  and,  after  join- 
ing another  rod  to  it,  pushes  this  also  into  the  duct.  Suc- 
cessive rods  are  joined  and  pushed  through  until  finally  the 
first  rod  reaches  the  next  man-hole.  A  rope  is  then  attached 
to  one  end  of  the  series  of  rods,  which  is  then  pulled  through, 
disjoining  the  rods  as  they  are  taken  out  of  the  duct.  Where 
the  ducts  are  smooth  and  comparatively  straight  this  process 
may  be  simplified  by  using  a  continuous  steel  wire  about  one- 
fourth  of  an  inch  in  diameter  in  place  of  the  rods.  It  is  a  good 
plan  to  attach  to  the  forward  end  of  this  wire  a  lead  ball,  which 
will  facilitate  it  in  riding  over  obstructions.  The  cable  reel  is  then 
placed  near  one  of  the  man-holes  in  such  manner  that  the  cable  will 


420 


AMERICAN    TELEPHONE   PRACTICE. 


pay  out  from  the  top  of  the  reel  instead  of  from  the  bottom. 
The  end  of  the  cable  is  then  attached  to  the  rope  and  started  into 
the  duct.  In  the  distant  man-hole  the  rope  is  led  over  one  or 
more  sheaves  suitably  arranged  on  upright  beams  placed  within 
the  man-hole  to  a  capstan  or  other  form  of  windlass  by  which  the 
cable  may  be  slowly  drawn  through  the  duct.  A  funnel-shaped 
shield  should  be  placed  at  the  mouth  of  the  duct  into  which  the 
cable  is  being  fed  for  protecting  the  sheath  against  the  sharp 
corners  at  the  entrance.  This  shield,  however,  is  not  a  sufficient 
protection  for  the  cable,  and  one  or  more  men  should  be  stationed 
in  the  man-hole  for  guiding  the  cable  into  the  duct.  The  best 
way  to  attach  the  rope  to  the  end  is  by  means  of  clamps  especially 
provided  by  the  cable  companies  for  this  purpose.  However,  if 
these  are  not  used,  a  secure  grip  may  be  had  upon  the  cable  end 
by  winding  several  strands  of  stout  iron  wire  in  opposite  direc- 
tions about  the  cable  sheath  for  a  distance  of  two  feet  from  its 
end.  An  eye  may  be  formed  in  this  wire  opposite  the  cable  end, 
to  which  the  rope  may  be  attached.  Particular  attention  should 
be  paid  to  the  sealing  of  the  cable  end  before  it  is  drawn  into  the 


Fig.  339. — Drawing  in  by  Steam  Power. 

duct,  as  ducts  are  always  moist,  due  so  sweating  of  the  interior 
walls. 

Where  a  large  amount  of  cable  is  to  be  drawn  in  the  method 
shown  in  Fig.  339  maybe  employed.  Instead  of  the  hand-oper- 
ated winch  or  windlass  a  three-and-one-half  horsepower  horizon- 
tal engine  and  capstan  mounted  on  a  low  wagon  is  used. 
By  suitable  gearing  the  engine  causes  the  capstan  to  revolve 
slowly.  This  method,  so  far  as  the  writer  is  aware,  has  been 
used  in  but  one  case,  that  being  in  the  recent  extensive  under- 
ground construction  work  in  St.  Louis  by  the  Bell  Telephone 
Company  of  Missouri.  It  is  said  that  with  this  contrivance 
a  speed  of  twenty-five  feet  of  cable  per  minute  is  easily  attained 
without  in  any  way  damaging  the  cable,  and  the  remarkably 


UNDERGROUND   CABLE    CONSTRUCTION.  421 

short  time  in  which  the  enormous  amount  of  cable  installed  by 
that  company  was  drawn  in  testifies  further  to  the  practical  value 
of  this  scheme. 

Cables,  in  passing  through  man-holes,  should  be  laid  around 
the  side  of  the  man-hole  and  supported  on  hooks  provided  for  that 
purpose.  Shields  formed  of  sheet  lead  or  of  heavy  felt  should  be 
placed  under  each  cable  just  at  the  point  where  it  emerges  from 
the  duct,  in  order  to  prevent  injury  of  the  sheath  at  that  point. 
Workmen  should  be  cautioned  against  needlessly  bending  cables 
while  working  in  ducts,  and  the  use  of  the  cables  in  place  of 
ladders  for  climbing  in  and  out  of  the  man-holes  should  be 
strictly  prohibited. 

As  much  slack  as  possible  should  be  left  in  the  man-hole,  in 
order  to  allow  room  for  subsequent  splicing  when  necessary. 
Trouble  is  frequently  experienced,  due  to  the  presence  of  gas 
in  the  man-hole,  and  care  should  always  be  exercised  before 
striking  a  match  or  taking  a  torch  into  a  man-hole,  to  make 
sure  that  all  gas  has  been  removed.  There  are  several  methods 
of  doing  this,  one  of  which  is  to  pump  the  gas  out  with  an 
inverted  umbrella  made  specially  for  the  purpose.  The  um- 
brella is  lowered  into  the  man-hole  while  closed  and  then  suddenly 
withdrawn,  this  opening  the  umbrella  and  lifting  out  the  gas. 
Another  way  of  clearing  man-holes  from  gas  is  to  place  a  cloth 
screen  above  the  man-hole  and  on  the  side  opposite  to  that  from 
which  the  wind  is  blowing.  The  wind  on  striking  the  screen 
is  deflected  downward,  thus  causing  an  eddy  which  removes 
the  gas  from  the  man-hole.  Very  serious  explosions  have  been 
caused  by  the  formation  of  gas  in  man-holes,  which  becomes  ig- 
nited either  by  an  electric  sp:irk  or  by  the  torch  of  a  workman. 

One  of  the  most  serious  difficulties  in  connection  with  un- 
derground cable  work  is  that  brought  about  by  electrolysis,  due 
to  the  action  of  stray  earth  currents,  usually  due  to  the  ground 
return  of  electric  railways.  It  is  found  that  the  electrolysis  oc- 
curs at  points  where  a  current  flowing  along  the  cable  sheath 
leaves  the  sheath  and  enters  the  ground.  At  this  point  oxy- 
gen is  liberated  which,  with  the  chemicals  in  the  earth,  rapidly 
corrodes  lead  sheaths.  Of  course,  the  construction  of  high-class 
conduits,  composed  of  insulating  material,  has  done  much  to- 
ward the  alleviation  of  this  trouble.  Frequent  tests  should  be 
made,  however,  on  all  cable  systems  to  determine  the  polarity 
of  the  cable  sheaths  with  respect  to  surrounding  conductors. 
The  tests  for  this  purpose  may  be  made  as  follows :  Two  brass 
rods  about  six  feet  long  should  be  provided,  each  having  a  steel 


422  AMERICAN    TELEPHONE   PRACTICE. 

contact  point  at  one  end.  Between  these  two  rods  should  be 
connected  by  flexible  wires  a  portable  voltmeter — one  reading 
to  five  volts  will  usually  be  found  most  suitable.  The  test 
should  be  made  at  the  man-holes,  these  being  the  only  avail- 
able points  for  reaching  the  cable.  One  of  the  steel  contact 
points  should  then  be  placed  in  firm  contact  with  the  cable 
sheath  and  the  other  into  contact  with  any  water  or  gas  pipes 
which  run  through  the  man-hole,  and  in  each  case  the  voltage 
should  be  noted,  not  only  in  amount  but  in  direction.  Read- 
"ing  should  also  be  taken  between  the  cable  sheaths  and  the 
rails  of  adjacent  electric  railroads,  and  to  whatever  underground 
structures  exist  in  the  immediate  vicinity.  It  is  evident  that 
where  the  cable  sheaths  are  negative  to  the  surrounding  con- 
ductors no  danger  will  exist,  as  this  would  indicate  that  the 
current  tended  to  flow  through  the  other  conductors  to  the 
sheath.  If,  however,  the  cable  is  found  positive  to  the  sur- 
rounding conductors,  the  matter  should  be  carefully  followed 
up  by  taking  readings  in  successive  man-holes.  By  these  means 
the  maximum  danger  point  can  be  located,  it  being,  as  a  rule, 
the  point  at  which  the  maximum  positive  difference  of  poten- 
tial exists.  At  this  point  the  cable  sheath  should  be  securely 
bonded  by  a  heavy  conductor  to  the  water  or  gas  pipe  or  to 
other  metallic  structures  that  are  in  the  vicinity.  These  bonds 
serve  to  allow  the  current  to  flow  from  the  cable  sheath  to  the 
other  conductors,  instead  of  forcing  it  to  find  circuit  through 
the  ground  or  through  the  walls  of  the  conduit.  In  some  cases 
the  only  remedy  has  been  to  run  separate  return  circuits  from 
the  maximum  danger  points  on  a  cable  directly  to  the  power- 
house from  which  the  troublesome  current  emanates.  All  of 
the  cable  sheaths  entering  a  man-hole  should  be  bonded  to- 
gether, the  usual  method  of  doing  this  being  to  brighten  the 
surface  of  the  lead  sheaths  and  to  bend  a  No.  10  B.  &  S.  cop- 
per wire  around  each  sheath,  afterwards  soldering  the  connec- 
tion. This  assures  the  fact  that  all  of  the  cable  sheaths  will 
be  at  an  equal  potential  and  that  whatever  bonds  are  run  for 
the  protection  of  one  sheath  will  afford  protection  for  all.  The 
method  of  bonding  to  a  gas  pipe  usually  adopted  is  as  follows : 
The  surface  of  the  pipe  is  brightened  for  a  space  of  about  three 
by  eight  inches  with  a  coarse  file.  This  surface  is  then  heated 
by  a  torch  and  tinned  with  ordinary  solder.  A  copper  plate 
about  three  by  seven  inches  previously  tinned  is  then  soldered 
to  the  gas  pipe,  after  which  the  bond  wire  leading  from  the 
cable  is  wound  into  a  flat  coil  and  soldered  to  a  copper  plate. 


UNDERGROUND   CABLE    CONSTRUCTION.  423 

In  bonding  to  a  water  pipe  it  is  impossible  to  heat  the  pipe 
sufficiently  to  make  it  take  solder  on  account  of  the  water  flow- 
ing within.  The  method  to  be  followed  is  to  provide  a  heavy 
wrought-iron  U-shaped  band  adapted  to  fit  snugly  around  the 
pipe.  The  ends  of  this  band  are  screw-threaded  and  pass  through 
a  yoke-piece  bent  to  fit  the  Upper  portion  of  the  pipe.  This 
yoke-piece  is  then  firmly  screwed  in  place  by  nuts,  the  surface  of 
the  pipe  and  the  interior  of  the  iron  clamp  having  previously  been 
thoroughly  brightened.  The  bond  wire  may  then  be  soldered  to 
this  yoke-piece  and  the  whole  device  smeared  with  asphalt  paint. 


CHAPTER  XXXIII. 

TESTING. 

TESTS  of  telephone  lines,  whether  of  bare  wire  on  poles  or  of 
overhead  or  underground  cables,  maybe  divided  into  two  general 
classes : 

First:  Those  which  are  for  the  determination  of  the  existence 
of  certain  conditions,  without  the  necessity  of  measuring  quanti- 
tatively the  extent  to  which  those  conditions  exist ;  in  other 
words,  rough  tests  for  the  determination  of  grounds,  crosses,  or 
breaks,  usually  made  with  instruments  such  as  the  magneto-bell, 
telephone  receiver  and  battery,  and  a  few  other  such  simple  but 
often  in  the  hands  of  an  experienced  person  most  effective 
instruments. 

Second  :  Those  for  not  only  determining  the  existence  of  cer- 
tain conditions,  but  also  for  their  quantitative  measurements. 
These  require  the  use  of  different  and  mor£  intricate  instruments, 
and  in  many  cases  the  operator  must  be  possessed  of  a  fair 
degree  of  mathematical  training  combined  with  an  ingenuity  for 
meeting  and  mastering  unusual  problems  that  arise  under  differ- 
•ent  conditions. 

The  magneto  testing  set  is  the  most  important  instrument  in 
making  tests  under  the  first  class.  Such  an  instrument  usually 
-consists  of  a  powerful  magneto-generator  so  wound  as  to  enable 
it  to  ring  its  own  bell  through  a  resistance  of  from  25,000  to 
75,000  ohms.  A  powerful  magneto-telephone  is  carried  on  the 
outside  of  the  case  in  suitable  clips,  and  may  be  switched  in 
circuit  alternately  with  the  generator  by  a  small  hand  switch. 
This  magneto-telephone  serves  as  both  transmitter  and  receiver, 
and  enables  the  lineman  or  other  party  to  communicate  from  a 
pole  top  or  man-hole  with  any  other  party  on  the  circuit.  Fre- 
quently these  sets  are  made  to  include  microphone  transmitter 
and  battery  ;  but,  inasmuch  as  the  instrument  is  seldom  if  ever 
used  to  talk  over  very  long  circuits,  the  extra  weight  of  these  is 
considered  in  most  cases  undesirable.  A  small,  inexpensive  gal- 
vanoscope  or  current  detector  will  also  prove  very  convenient. 

In  testing  for  a  ground  on  a  wire,  whether  it  be  in  a  cable  or 
bare,  and  on  poles,  make  sure  that  the  far  end  of  the  line  is  open 


7 '£  STING. 


425 


and  then  connect  one  terminal  of  the  magneto-bell  to  the  near 
end  of  the  line  and  ground  the  other  terminal.  The  ringing  of 
the  bell  would  seem  to  indicate  that  the  circuit  was  complete 
and  the  line  grounded  in  this  case,  but  this  is  not  always  true, 
and  this  test  must  therefore  be  relied  on  only  with  caution.  The 
static  capacity  of  a  long  line  or  of  a  comparatively  short  length 
of  cable  will  often  allow  enough  current  to  pass  to  and  from  the 
line  in  charging  and  discharging  to  ring  the  magneto-bell. 

For  testing  out  local  work  where  there  is  no  room  for  this 
capacity  effect,  the  magneto-bell  is  invaluable. 

A  more  reliable  means  of  making  tests  for  grounds  or  crosses 
is  to  connect  the  current  detector  in  series  with  several  cells  of 
battery  and  to  ground  one  terminal.  Then  with  the  other  ter- 


Fig.  340. — Receiver  Test  for  Crosses  and  Grounds. 


minal  make  contact  with  the  near  end  of  the  line.  A  kick  of  the 
needle  will  take  place  in  any  event  on  closing  the  circuit,  due  to 
the  current  flowing  to  charge  the  line,  but  a  permanent  deflection 
will  indicate  a  ground. 

In  testing  for  a  cross,  as  for  instance  with  some  other  wire  in 
the  line  or  cable,  one  terminal  of  the  magneto-bell  or  the  gal- 
vanoscope  and  batteries  should  be  connected  to  the  wire  under 
test  and  the  other  to  all  the  other  wires  in  the  same  lead,  for 
which  purpose  they  are  bunched.  In  case  it  is  not  convenient 
to  bunch  them,  however,  the  test  may  be  made  between  the  sus- 
pected line  and  each  of  the  others  in  succession. 

Another  and  perhaps  still  more  simple  method  for  determining 
a  cross  or  ground  is  one  described  in  Roebling's  pamphlet  on 
Telephone  Cables,  and  illustrated  in  Fig.  340,  as  applied  to  the 
testing  of  a  cable  before  it  has  been  unreeled. 


426  AMERICAN   TELEPHONE  PRACTICE. 

N  represents  the  near  end,  and  F  the  far  end  of  the  wire 
being  tested.  B  is  a  battery,  of  about  three  cells.  T  is  an 
ordinary  telephone  receiver.  The  wire,  N  F,  is  carefully  sepa- 
rated from  all  the  others  at  each  end. 

At  the  near  end  all  the  wires  are  stripped  of  insulation  and, 
except  the  one  under  test,  are  connected  together  and  also  with 
the  sheath.  The  wire,  C,  connects  the  sheath  to  one  side  of  bat- 
tery, B,  and  the  other  side  of  battery  is  connected  to  one  side  of 
telephone  receiver,  T.  The  testing  man  rapidly  taps  with  the 
wire,  N  F,  the  unoccupied  binding  post  of  the  receiver,  T.  The 
first  tap  will  produce  in  the  receiver  a  distinct  click,  and  if 
the  cable  is  long  there  may  possibly  occur  a  second  faint  click, 
but  if  the  wire,  N  F,  is  perfectly  insulated,  no  more  sound  in  the 
telephone  will  follow  the  tapping.  If,  however,  the  wire,  N Ft  is 
crossed  with  any  wire  in  the  cable,  or  with  the  sheath,  every  tap 
will  be  followed  by  a  distinct  click,  and  if  there  is  moisture  in  the 
paper,  making  a  partial  connection,  clicking  sounds  will  occur, 
which  are  loud  or  faint,  according  to  the  amount  of  moisture 
present. 

The  philosophy  of  this  method  of  testing  is  very  simple,  and 
serves  to  make  the  operation  more  readily  understood. 

When  the  wire,  N  F,  is  first  connected  to  the  battery,  it  be- 
comes charged.  During  the  process  of  charging  a  current  flows 
into  the  wire  and  passes  through  the  coil  of  the  receiver  and 
causes  the  click.  If  the  wire  is  well  insulated,  the  second  tap, 
immediately  following,  finds  it  charged,  or  nearly  so,  and  there 
is,  therefore,  no  click,  or  a  very  faint  one.  If,  on  the  contrary, 
the  wire  under  test  is  crossed  with  any  of  the  other  wires,  or 
imperfectly  insulated  from  them,  or  from  the  sheath,  the  wire 
will  immediately  discharge  itself  through  the  cross  to  the  other 
wires  and  the  sheath,  and  there  will  be  a  flow  of  current  at  every 
tap,  and  consequently  a  continuous  clicking.  If  a  conductor  in 
a  perfectly  insulated  cable  is  very  long,  two  or  three  taps  or  a 
long  first  contact  may  be  necessary  to  charge  it  completely. 

If  the  cable  is  in  place  or  if  it  is  a  bare  aerial  line  that  is  being 
tested  this  same  method  may  be  used.  In  case  of  a  new  cable  it 
is  well  to  test  every  wire  in  this  manner,  and  therefore  the  wire, 
N  F,  should  be  put  aside  and  another  slipped  out  of  the  bunch 
and  tested  in  the  same  way,  and  so  on  until  all  have  been  gone  over. 

If  any  of  them  are  found  to  be  in  trouble,  it  is  well  to  care- 
fully inspect  the  exposed  ends  to  be  sure  they  are  properly 
cleared  from  each  other  and  from  the  sheath.  If  it  is  still  found 
to  be  defective,  it  should  be  plainly  tagged. 


TESTING. 


427 


In  the  manner  just  described,  twenty-five  minutes  with  two 
men  should  be  ample  time  for  testing  one  hundred  wires,  the 
testing  operator  listening  and  his  helper  attending  to  the  connec- 
tion of  the  different  wires  at  N. 

For  this  test  as  well  as  many  others  it  is  very  convenient  to 
use  a  regular  operator's  receiver  and  head  band,  as  it  will  save  the 
tester  a  very  tired  arm  at  the  end  of  a  long  test.  As  a  matter  of 
fact,  the  receiver  is  little  appreciated  as  a  testing  instrument.  A 
very  convenient  set  is  formed  by  a  watch-case  receiver  and  head 
band,  and  two  small-sized  cells  of  dry  battery,  strapped  together 
so  as  to  be  carried  in  the  coat  pocket.  The  receiver  and  battery 
are  connected  in  series,  the  free  terminals  of  the  circuit  being 
formed  by  flexible  cords  about  four  feet  long.  These  cords 
should  terminate  in  convenient  clips,  or  contact  points  adapted 


Fig.  341. — Continuity  Test. 

to  make  contact  with  the  wires  to  be  tested.     This  arrangement 
leaves  both  hands  free  at  all  times,  and  is  wonderfully  sensitive. 

The  continuity  test,  or  test  for  broken  wires,  may  be  made 
with  the  same  simple  instruments.  The  wires  to  be  tested  should 
all  be  grounded  or  connected  to  a  return  wire  at  the  far  end.  At 
the  near  end,  one  pole  of  a  magneto-bell,  or  of  the  battery  and 
galvanoscope,  or  of  the  receiver,  should  be  connected  to  ground  or 
the  return  wire  and  the  other  terminal  connected  successively  to 
the  terminals  of  the  line,  which,  of  course,  should  all  be  separated. 
A  ring  in  the  case  of  the  magneto,  or  a  permanent  deflection 
of  the  needle  in  the  case  of  the  galvanoscope,  or  a  continuous 
clicking  in  the  receiver,  will  indicate  that  the  wire  is  continuous. 
The  same  precaution  as  previously  pointed  out  must,  however,  be 


428  A  AMERICAN   TELEPHONE   PRACTICE. 

observed  with  the  magneto-bell.  This  same  test  for  continuity 
is  well  illustrated  in  Fig.  341,  in  which  case  a  vibrating  bell  in- 
stead of  the  receiver  or  galvanoscope  is  used. 

In  testing  a  cable  all  defective  wires  should  be  marked 
"crossed,"  "grounded,"  or  "  broken  "  at  the  end  at  which  they 
are  tested.  The  corresponding  ends  of  the  tagged  wires  at  the 
other  end  of  the  cable  should  then  be  found  and  similarly  marked. 
If  there  are  not  the  requisite  number  of  good  wires  in  a  new 
cable  it  should  be  rejected. 

Lead-covered  cables,  manufactured  by  reliable  firms,  are  always 
subjected  to  a  much  severer  test  than  it  is  possible  for  the  pur- 
chaser to  give  them  before  they  leave  the  factory.  It  is  there- 
fore considered  by  many  as  unnecessary  to  make  a  test  on  new 
cable  on  reels,  purchased  from  reputable  firms,  unless  some  injury 
in  shipment  is  suspected. 

It  is  often  desirable  to  be  able  to  pick  out  a  certain  wire  at 
some  intermediate  point  in  an  open  cable,  or  in  a  large  bunch  of 
insulated  wires,  in  order  to  establish  a  branch  connection.  This 
is  easily  done  by  the  foregoing  methods  if  the  cable  is  to  be  cut, 
but  frequently  this  is  not  the  case.  It  may  be  done  without 
cutting  by  the  following  simple  method  :  Ground  the  wire  or 
wires  desired  at  the  distant  end,  being  sure  that  these  wires  are 
free  from  all  the  others  at  both  ends.  Then  having  loosened  the 
bunch  of  wires  at  the  point  at  which  the  branch  is  to  be  taken 
off,  test  each  by  means  of  a  needle-pointed  instrument,  con- 
nected to  ground  through  a  bell  or  receiver  and  battery.  The 
needle-point  can  readily  pierce  the  insulation  and  make  good 
contact  with  the  conductor  within.  A  knowledge  of  this  very 
simple  test  will  often  save  an  immense  amount  of  trouble. 

In  the  second  class  of  tests — that  is,  those  requiring  quantitative 
measurements — there  are  three  distinct  subdivisions,  which  are  as 
follows :  Tests  for  resistance  or  conductivity,  tests  for  capacity, 
and  tests  for  insulation.  Tests  for  the  location  of  faults  in  lines 
always  depend  on  the  application  of  one  or  more  of  these. 

There  are  three  principal  methods  of  making  resistance  tests: 
First,  by  the  use  of  a  Wheatstone  bridge,  which  is  accurate  for  all 
resistances  except  those  very  large  or  those  very  small.  Second, 
the  fall  of  potential  method,  which  is  of  value  in  measuring  very 
small  resistances,  as  of  a  large  conductor,  such  as  a  trolley-wire  or 
heavy  feeder.  This  method  has  little  use,  therefore,  in  telephone 
work  where  all  conductors  are  comparatively  small.  Third,  by 
the  use  of  a  sensitive  galvanometer  in  series  with  a  battery. 
This  method  is  the  most  accurate  for  the  determination  of 


TESTING. 


429 


extremely  high  resistances  and  is,  therefore,  of  great  use  in  meas- 
urements of  insulation  resistance. 

For  general  resistance  measurements  the  Wheatstone  bridge 
is  the  most  suitable,  being  very  accurate  and  exceedingly  simple 
in  manipulation.  In  order  to  appreciate  the  possibilities  of  this 
instrument  its  underlying  principles  should  be  understood.  In 
Fig.  342,^,^,^,  and  X represent  resistances.  G  is  a  galvanometer 
or  instrument  for  detecting  ^the  flow  of  current.  The  four 


Fig.  342. — Diagram  of  Wheatstone  Bridge. 


resistances  are  connected  together  as  shown,  the  galvanom- 
eter being  connected  in  the  "bridge"  between  the  junctures  of 
A  and  R,  and  B  and  of  X.  A  battery,  B ',  is  connected  between  the 
junctures  of  A  and  B,  and  of  R  and  X.  Each  resistance,  A,  B,  R, 
and  X,  forms,  what  is  termed  an  arm  of  the  bridge. 

The  two  fundamental  laws  upon  which  the  action  of  the  bridge 
is  based  may  be  stated  as  follows  : 

1.  No  current  will  flo^v -between  points  of  equal  potential;  and 

2.  The  drop  in  potential  along  the  various  parts  of  a  conductor 
is  proportional  respectively  to  the  resistances  of  those  parts. 

Referring  again  to  the  diagram,  it  is  evident  that  a  current 
from  the  battery  flows  to  the  point,  ^,  where  it  divides,  part  flow- 
ing through  A  R  and  part  through  B  X,  after  which  they  unite 
and  pass  to  the  negative  pole  of  the  battery.  But  what  of  the 
galvanometer?  Evidently  by  Rule  I  the  only  time  at  which  no 
current  will  pass  through  it  will  be  at  the  time  when  the  points, 
f  and  h,  are  at  the  same  potential.  By  Rule  2  these  points 
will  be  at  the  same  potential  only  when  A  bears  the  same  re- 
lation to  R  as  J3  does  to  X. 


430  AMERICAN    TELEPHONE  PRACTICE. 

That  is 

A  :   R  ::  B   :   X,  or,  by  alternation, 

A  R 

B  ~~  X  ' 

A  little  algebra  will  render  the  above  evident  if  not  so  already. 
Call  a  the  drop  of    potential  between  the  points  e  and   z,  / 
that  between  e  and/,  and  c  that  between  e  and  h. 
Then 

b    :   a  ::  A    :    A  +  R  by  Rule  2. 


Similarly 


B 

'-  a' 


For  a  condition  of  equal  potentials  at  /and  h  so  that  no  cur- 
rent will  flow  through  the  galvanometer,  b  must  =  c. 
Then 

A  B 


A  +  R  B  +  X 

whence  :  AB  -f  ^Ar  =  ^  +  BR, 

and  ^A"  =  BR. 

Dividing  by  BX,  we   have 

A  R 

B  JT 

which  is  the  equation  of  the  ratios  between  the  resistances  of  the 
arms  of  the  bridge,  to  insure  no  flow  of  current  through  the 
galvanometer. 

The  resistance  to  be  measured  forms  the  arm  X  of  the  bridge, 
and  in  order  to  determine  its  value  the  resistances  in  the  various 
arms  are  adjusted  till  no  current  flows  through  the  galvanom- 
eter. Then  the  equation  just  derived  holds  good  and  may  be 
solved  for  X, 

thus  X  =  J*—  R. 
A 

The  arms  A  and  B  are  best  termed  the  "  ratio  arms  "  of  the 
bridge  and  arm  R  the  rheostat  arm. 

In  commercial  forms  of  the  Wheatstone  bridge,  A  and  B  are 
usually' so  arranged  that  each  may  be  given  the  values,  10,  100, 
and  1000  ohms,  and  in  some  cases  I  ohm  and  10,000  ohms  also. 


TESTING.  43  J 

The  ratio  arms,  A  and  B,  may  therefore  be  adjusted  to  bear  any 
convenient  ratio  to  each  other  from  -  -  to  IQQQ  ,  or,  in 

1000  10 

some  instances,  from  -  to  -  L2^2°_  .     The     rheostat    arm 

10,000  I 

is  in  reality  a  rheostat  capable  of  being  adjusted  to  any  value 
from  I  to  about  11,000  ohms. 

In  some  bridges  a  sealed  battery  is  furnished  with  and  forms 
a  part  of  the  instrument.  In  those  having  no  battery,  suitable 
binding  posts  are  provided,  usually  marked  BB,  between  which 
the  battery  may  be  connected.  Other  binding  posts,  usually 
marked  XX,  are  furnished  for  connecting  the  terminals  of  the 
unknown  resistance  to  be  measured. 

Two  keys  are  usually  furnished,  one  in  the  battery  circuit  and 
the  other  in  the  galvanometer  circuit.  Each  keeps  its  circuit 
normally  open. 

The  operation  of  the  bridge  is  very  simple.  First  some  ratio 
between  the  arms  A  and  B  is  determined  upon.  The  battery  is 
then  connected  between  the  proper  binding  posts,  and  likewise 
the  resistance  to  be  measured  is  connected  between  its  binding 
posts. 

The  battery  key  is  first  depressed  and  then  the  galvanometer 
key.  A  deflection  of  the  galvanometer  needle  will  take  place 
which  by  its  direction  will  after  a  few  trials  show  whether  the 
resistance  in  the  rheostat  arm  is  too  great  or  too  small.  The 
rheostat  is  adjusted  accordingly  until  the  galvanometer  needle 
shows  no  deflection  upon  the  operation  of  the  keys.  We  then 
know  that  our  equation 

4-  =  4-  h°lds 

£>  A. 

and  consequently 

X=         *  *' 


That  is,  the  unknown  resistance  is  equal  to  the  ratio  between  B 
and  A  multiplied  by  the  resistance  in  the  adjustable  arm. 

Considerable  judgment  may  be  exercised  in  the  choosing  of 
the  appropriate  ratio  in  the  ratio  arm  to  obtain  the  greatest 
accuracy.  Obviously  if  a  very  high  resistance  is  to  be  measured 
the  ratio  should  be  large,  and  vice  versa. 

In  bridges  having  resistances  of  10,  100,  and  1000  ohms  in  the 


432  AMERICAN   TELEPHONE  PRACTICE. 

ratio  arms,  the  following  values  in  arms  A  and  B  will  give  the 
best  results : 

Resistance  to  be  measured.  A  arm.  B  arm. 

Under  100  ohms, 1000  10 

100  to  1000  ohms,      .....  1000  100 

1000  to  10,000  ohms,          ....  1000  1000 

10,000  to  100,000  ohms,     .         .         .         .  100  1000 

100,000  to  1,000,000  ohms,         ...  10  1000 

As  to  the  accuracy  of  measurements  attainable  by  the  use  of 
the  Wheatstone  bridge,  the  following  table  represents  the  claim 
of  one  reliable  manufacturer: 

.01  of  an  ohm  to  an  accuracy  of  I  per  cent. 

•r  it  «  t<  «  «      T/  «  « 

I  ohm  "  "  "  ^  "  " 

10  ohms  "  "  "  \  "  " 

100  "  "  "  "  ?/8  " 

1000  "  "  "  "  */8  " 

10,000  "  "  "  "  I  "  " 

100,000  "  "  "  "  #  "  " 

1,000,000  "  "  "         "  5     "      " 

If  using  the  no  volt  lighting  circuit  as  battery  power  I  meg- 
ohm may  be  measured  accurate  to  j£  per  cent. 

There  is  no  doubt  but  that  with  a  well-made  bridge  with  a 
sensitive  galvanometer,  these  results  may  be  equaled  if  not  sur- 
passed. Great  care  must  be  taken  in  using  a  voltage  as  high  as 
no,  as  there  is  danger  of  burning  out  the  coils.  Such  high  volt- 
age should  be  used  only  in  measuring  very  high  resistances,  and 
the  ratio  arms  should  be  adjusted  to  give  as  high  a  multiplying 
ratio  as  possible. 

A  particular  form  of  bridge  which  has  come  into  extensive  use 
in  this  country  and  which  possesses  several  unique  features  is 
shown  complete  in  Fig.  343  and  in  plan  view  in  Fig.  344. 

The  various  adjustments  of  the  arms  are  accomplished  by 
placing  plugs  in  the  various  holes  between  the  brass  blocks 
arranged  in  rows  as  shown  in  the  latter  figure.  Between  each 
successive  pair  of  blocks  are  arranged  resistance  coils  having  the 
resistances  in  ohms  designated  on  the  plan.  Placing  a  plug  in  a 
hole  between  two  blocks  short-circuits  the  resistance  connected 
between  those  two  blocks.  The  rheostat  arm  of  this  bridge  is 
represented  by  the  top  and  bottom  row  of  blocks,  and  if  all  plugs 


TESTING. 


433 


are  removed  the  resistance  in  this  arm  will  amount  to  11,110 
ohms.  The  ratio  arms  A  and  B  are  represented  by  the  left-  and 
right-hand  halves  respectively  of  the  center  row.  A  galva- 


Fig.  343. — Portable  Testing  Set. 


nometer  and  suitable  battery,  together  with  battery  and  galva- 
nometer keys,  are  all  mounted  in  a  carrying  case  as  shown. 

The   connections  of  this  instrument  are  indicated  in  Fig.  344, 


Fig.  344. — Plan  of  Portable  Testing  Set. 

and  are  3&  follows :  The  top  row  of  blocks  is  connected  to  the 
bottom  r*>w  by  a  heavy  copper  bar  joining  the  right-hand  blocks. 
This  connection  is  made  very  heavy  so  as  to  interpose  no  extra 
resistance  in  the  rheostat.  On  the  rheostat  formed  by  the 


434 


AMERICAN    TELEPHONE   PRACTICE. 


upper  and  lower  rows  of  blocks  any  resistance  from  i  to 
1 1, no  ohms  may  be  obtained,  the  resistance  being  added  by 
leaving  out  plugs.  The  lower  left-hand  block  of  the  rheostat  is 
connected  to  the  lower  binding  post,  D,  forming  one  terminal  of 
the  unknown  resistance.  The  upper  post,  C,  forming  the  other 
terminal  of  the  unknown  resistance,  is  connected  to  block,  X, 
which  block  is  also  joined  to  the  galvanometer  key.  The  block, 
R,  is  connected  to  the  upper  left-hand  block  of  the  rheostat. 
The  end  blocks  of  the  middle  row  are  connected  together  and 
to  the  -+-  terminal  of  the  battery.  The  —  terminal  of  the  bat- 
tery is  connected  through  the  battery  key  to  the  lower  left-hand 
end  of  the  rheostat.  One  galvanometer  terminal  is  connected 
directly  to  the  block,  R,  the  left-hand  block  of  the  rheostat, 
and  to  the  back  contact  of  the  galvanometer  key.  The  other 
galvanometer  terminal  is  connected  through  the  key  to  the 
block,  X. 

By  carefully  following  out  these  connections  it  will  be  apparent 
that  the  parts  as  connected  form  three  arms  of  a  Wheatstone 
bridge,  the  fourth,  of  course,  being  the  unknown  resistance 


•&•  345- — Circuits  of  Portable  Testing  Set. 

joined  to  the  line  posts.     This  is  shown  diagrammatically  in  Fig. 
345,  where  the  corresponding  parts  are  similarly  lettered. 

It  will  be  noticed  that  this  latter  figure  is  practically  the  same 
as  Fig.  342,  with  the  addition  of  the  center  blocks,  A,  B,  X,  R, 
forming  a  sort  of  commutator.  The  object  of  this  arrangement 
is  to  make  it  possible  to  reverse  the  connections  of  arms,  A  and 
B,  with  R  and  X.  Thus  with  the  plugs  in  the  position  shown  by 
the  black  dots,  the  connection  is  precisely  as  shown  in  Fig.  342, 
A 


and   the   equation      -  =  JiL  holds  true. 
B       X 


If,  however,  the  plugs  are 

inserted  in  the  holes  on  the  other  diagonal,  arm,  A,  will  be  con- 
nected to  arm,  X,  and  arm,  B,  to  arm,  R,  and  the  equation  of 

the  bridge  will  be  -—  =      -^L 


TESTING.  435 

The  bridge  arms,  A  and  B,  have  not  the  same  range  of  resist- 
ances in  this  bridge,  A  having  only  i,  10,  and  100  ohm  coils, 
while  the  resistances  of  ^are  10,  100,  and  1000  ohms.  Therefore, 
if  a  ratio  of  looo  to  I  for  measuring  large  resistances  is  desired, 
the  plugs  are  inserted  in  the  commutator  along  the  arrow  H 
(Fig.  344) ;  while  an  opposite'arrangement  of  the  plugs  along  the 
arrow  L  will  give  a  ratio  of  I  to  1000  for  very  small  resistances. 
In  this  bridge  the  galvanometer  key  is  so  arranged  as  to  short- 
circuit  the  galvanometer  while  the  key  is  up. 

The  galvanometers  usually  furnished  with  the  complete 
bridges  consist  of  a  needle  so  mounted  as  to  swing  freely  in  a 
horizontal  plane.  This  needle  is  given  a  tendency  to  point  in 
one  direction  sometimes  by  the  action  of  the  earth's  magnetic 
field  and  sometimes  by  the  field  of  a  powerful  permanent  mag- 
net. By  causing  the  current  through  the  bridge  wire  to  flow 
through  a  coil,  either  stationary  and  surrounding  the  needle,  or 
movable  and  carried  on  the  needle,  the  needle  is  caused  to 
swerve  from  its  normal  position  and  to  place  itself  at  right-angles 
to  the  lines  of  force  due  to  the  permanent  field.  The  deflection 
of  the  needle  is  great  or  small  according  to  the  strength  of  the 
current,  and  to  the  right  or  left  according  to  the  direction  of  the 
current. 

In  many  of  the  tests  to  be  described  later  a  galvanometer  of 
greater  sensitiveness  is  required,  and  some  form  of  reflecting  in- 
strument is  used.  In  these  the  needle  carries  a  small  circular 
mirror,  which  reflects  a  spot  of  light  from  a  lamp  or  some  other 
source  against  a  scale.  In  this  arrangement  every  movement  of 
the  needle  causes  the  spot  of  light  to  move  along  the  scale,  and 
a  little  consideration  will  show  that  the  angle  through  which  the 
reflected  ray  of  light  moves  is  double  that  through  which  the 
needle  travels.  Thus  this  reflected  ray  of  light  serves  as  a 
needle  of  any  desired  length,  and  has  the  advantages  of  magnify- 
ing the  angular  movement  of  the  needle  to  twice  its  real  amount, 
and  of  possessing  no  mass,  and  therefore  no  inertia. 

The  two  galvanometers  used  to  the  greatest  extent  for  quanti- 
tative measurements  in  practical  work  are  the  Thomson  and  the 
D'Arsonval. 

The  Thomson  galvanometer  is  made  in  a  great  variety  of 
forms.  The  needle  consists  of  several  very  light  bar-magnets 
arranged  side  by  side  and  with  opposing  poles  together,  so  that 
the  directive  influence  of  the  earth's  field  shall  be  very  slight. 
The  needle  is  directly  attached  to  a  small  silvered  glass  mirror, 
and  is  suspended  within  the  coil  or  coils  by  means  of  a  silk  or 


436 


AMERICAN   TELEPHONE  PRACTICE. 


quartz*  fiber.  The  current  to  be  measured  is  passed  through  the 
coils,  and  the  magnetic  field  set  up  thereby  causes  the  needle  to 
swerve  from  its  normal  position.  The  Thomson  galvanometer  is 
used  in  the  most  delicate  tests,  and  is  essentially  a  laboratory  in- 
strument. It  has  the  disadvantage  of  being  affected  to  such  an 
extent  by  external  magnetic  fields  as  to  render  its  use  impos- 
sible in  many  cases.  A  passing  street  car  or  variations  in  the 


Fig.  346. — D'Arsonval  Galvanometer. 

current  flowing  in  a  neighboring  circuit  will  cause  the  needle  to 
swing  violently,  thus  making  accurate  work  out  of  the  question. 
These  disadvantages  may  be  overcome  to  some  extent  by  in- 
closing the  galvanometer  in  a  heavy  iron  case — such  as  an  old 
safe — but  they  tend  to  make  it  a  very  undesirable  instrument 
for  portable  work.  Where  the  instrument  can  be  permanently 
set  up  and  properly  guarded,  it  is  unequaled  for  delicacy  and 
accuracy. 

For  nearly  all  practical  engineering  work,  the  D'Arsonval  gal- 
vanometer is  sensitive  enough,  and  has  the  advantage  of  being 
much  more  convenient  for  general  work.  In  this  the  needle  is  a 
coil  instead  of  a  permanent  magnet,  and  is  suspended  within  the 


TESTING. 


437 


field  of  a  powerful  permanent  magnet  instead  of  in  a  coil.  The 
needle  carries  a  mirror,  as  in  the  Thomson  instrument.  The 
current  to  be  measured  is  passed  through  the  coil,  and  as  this 
coil  lies  in  the  field  of  the  permanent  magnet,  a  rotation  of  the 
coil  ensues,  the  action  being  identical  with  that  which  causes 
the  armature  of  an  electric  motor  to  revolve. 

In  Fig.  346  is  shown  a  much-used  form  of  D'Arsonval  gal- 
vanometer made  by  Queen  &  Co.,  Philadelphia.  The  field  is  built 
up  of  a  number  of  horizontal  permanent  magnets,  between  the 


Fig.  347.— Suspension  of  D'Arsonval  Galvanometer. 

poles  of  which  is  suspended  the  needle.  The  needle  system 
is  shown  in  detail  in  Fig.  347.  It  consists  of  a  coil  of  wire,  W, 
wound  on  a  boxwood  frame,  D,  and  supported  by  means  of  the 
flat  phosphor-bronze  filament,  A,  from  the  torsion  pin,  E.  The 
current  is  led  in  by  means  of  the  torsion  pin,  E,  and  suspension 
wire  to  the  coil ;  thence  to  the  spiral  spring,  B,  and  by  means  of 
the  bottom  contact  out  to  the  external  circuit.  A  ring,  F,  is 


438  AMERICAN    TELEPHONE   PRACTICE. 

joined  above  the  coil  frame,  and  another,  G,  below  the  coil  frame. 
These  are  normally  a  sufficient  distance  apart  to  enable  the  sys- 
tem to  swing  freely,  but  when  packing  for  transportation  the 
torsion  head  may  be  pressed  down  until  the  rings  above  men, 
tioned  firmly  clamp  the  coil.  In  this  condition  it  will  withstand 
shipment  satisfactorily.  To  the  right  is  shown  the  coil  more 
clearly.  The  two  points,  U  and  /,,  have  soldered  to  them  the 
ends  of  the  coil,  W.  The  mirror  is  shown  at  C. 

The  great  advantage  of  the  D'Arsonval  galvanometer  is  that 
It  is  unaffected  by  variations  in  the  external  magnetic  field.  It 
may  even  be  used  close  to  dynamo  machinery  without  being 
sensibly  affected. 

In  order  to  read  the  deflection  produced  by  a  current,  in  any 


Fig.  348. — Scale  and  Telescope. 

form  of  reflecting  galvanometers,  two  methods  may  be  em- 
ployed. One  is  to  cause  the  needle  to  reflect  a  spot  of  light 
from  a  stationary  source,  upon  a  horizontal  scale,  and  by  watch- 
ing the  movement  of  the  spot  the  number  of  scale  divisions  de- 
flection may  be  accurately  determined.  Another  and  better 
way  is  to  focus  a  telescope  on  the  mirror,  in  such  manner  that 
the  horizontal  scale  will  be  visible  in  the  telescope.  The  mirror 
in  its  movements  will  reflect  different  portions  of  the  scale  into 
the  telescope,  and  the  deflection  may  thus  be  observed  with 
great  precision.  When  this  method  is  used  the  numbers  on  the 
scale  should  be  reversed,  in  order  to  appear  normal  in  the  tele- 
scope., Fig.  348  shows  a  telescope  and  scale  as  arranged  for 
this  purpose. 

Complete   testing   sets,   containing    reflecting   galvanometers, 


TESTING.  439 

bridges,  batteries,  keys,  and  other  accessories,  are  frequently 
mounted  in  one  case,  and  so  arranged  as  to  fold  within  small 
compass  when  not  in  use.  This  arrangement  is  very  convenient, 
but  has  one  disadvantage — the  manipulation  of  the  keys  and 
plugs  jar  the  box  to  such  an  extent  as  to  make  the  readings 
on  the  galvanometer  unreliable.  The  separately  mounted  gal- 
vanometer is  therefore  in  general  to  be  preferred.  Of  course 
this  applies  only  to  reflecting  galvanometers. 

It  is  frequently  found  that  a  current  that  it  is  desired  to 
measure  is  so  large  that  it  sends  the  spot  of  light  completely  off 
the  scale,  thus  rendering  the  measurement  of  the  deflection  im- 
possible. In  order  to  increase  the  range  of  the  galvanometer  so 
as  to  make  it  available  for  measuring  both  large  and  small  cur- 
rents, certain  resistances  called  shunts  may  be  placed  in  parallel 
with  the  galvanometer  coil  as  in  Fig.  349.  The  resistance  of 


S 
Fig.  349.  —  Galvanometer  and  Shunt. 

the  shunt  being  known,  it  is  easy  to  calculate  the  amounts  of 
the  currents  that  pass  through  the  galvanometer  coil  and  the 
shunt. 

Calling  Rs  the  resistance  of  the  galvanometer,  R*  that  of  the 
shunt,  Ig  the  current  through  the  galvanometer,  7S  that  through 
the  shunt,  and  /  the  total  current  through  both,  then 

7=4+  /.. 

Also  when  E  is  the  difference  of  potential  between  the  com- 
mon terminals  of  the  galvanometer  and  shunt, 

E  ,         E 

7g  =  _   and   78  ==  _. 


E  =  LR.-.--I.  Rs.     Hence  /.  =  ^- 


Substituting  this  value  of  7S,  in  the  first  equation,  we  have 
~  R*  e  \         ~RJ  X* 

7?      i      T2 

The  quantity  -    — — —  is  called  the  multiplying  power  of  the 


440  AMERICAN    7^ELEPHONE   PRACTICE. 

shunt,  because  it  represents  the  number  by  which  the  current 
through  the  galvanometer  must  be  multiplied,  in  order  to  give 
the  value  of  the  current  being  measured. 

Shunt  boxes  are  usually  provided  for  a  given  galvanometer 
with  a  number  of  coils  specially  arranged  to  give  such  con- 
venient values  of  the  multiplying  powers,  as  10,  100,  and  1000. 
For  this  purpose  the  various  coils  of  the  shunt  box  have  resist- 
ances of  |,  -jfo,  and  -rrj-g-of  the  resistance  of  the  galvanometer. 

To  better  show  this  relation,  assume  that  a  multiplying  power 
of  1000  is  desired,  then 


I000  = 


-  RK  -  R.. 


iooo  —   i       999 

A  commercial  form  of  shunt  box  is  shown  in  Fig.  350,  the 
various  multiplying  values  of  the  shunt  being  obtained  by  plug- 
ging the  block  corresponding  to  the  multiplying  power  desired. 


Fig.  350. — Shunt  Box. 

For  moderate  deflections,  the  current  traversing  the  coils  of  a 
reflecting  galvanometer  may,  without  sensible  error,  be  taken  as 
proportional  to  the  deflection  of  the  spot  of  light  on  the  scale, 
or  to  the  deflection  read  through  the  telescope.  The  current  is 
of  course  inversely  proportional  to  the  total  resistance  of  the 
circuit,  and  from  this  it  follows  that  the  deflections  are  inversely 
proportional  to  the  resistance.  This  fact  enables  the  galva- 
nometer to  be  used  for  measuring  unknown  resistances  by  com- 
paring the  deflection  obtained  when  a  given  E.  M.  F.  acts 
through  a  known  resistance  with  that  obtained  when  the  same 
E.  M.  F.  acts  through  an  unknown  resistance. 

The  general  method  of  measuring  resistances  by  the  use  of  a 


TESTING.  44i 

galvanometer  is  to  note  the  deflection  obtained  with  a  given 
battery  and  a  known  resistance  in  the  circuit,  and  from  this  to 
compute  what  is  called  the  working  constant.  This  working 
constant  may  be  defined  as  the  number  of  scale  divisions  deflection 
that  ivould  be  obtained  by  causing  the  current  from  the  given  bat- 
tery to  pass  through  the  galvanometer  and  a  resistance  of  one 
megohm.  Of  course  such  a  deflection  as  this  can  exist  in  our 
imagination  only,  but  it  serves,  nevertheless,  as  a  convenient 
standard  upon  which  to  base  our  calculations.  Having  obtained 
the  working  constant,  a  reading  is  taken  of  the  deflection  pro- 
duced by  passing  the  battery  current  through  the  galvanometer 
in  series  with  the  unknown  resistance.  As  the  deflections  are 
inversely  proportional  to  the  resistances,  the  unknown  resist- 
ance is  then  readily  computed. 

If  measurements  of  comparatively  low  resistance  are  to  be 
made,  then  the  resistance  of  the  battery  and  of  the  galva- 
nometer must  be  taken  into  consideration  as  well  as  that  of  the 
resistance  placed  in  circuit  with  them,  but  as  the  measurements 
here  considered  will  be  those  of  very  high  resistances  only,  the 
resistance  of  the  battery  and  of  the  galvanometer  may  be  neg- 
lected. For  the  purpose  of  taking  the  constant,  connections 
are  made  as  shown  in  Fig.  351,  where  B  is  the  battery,  G  the 


Fig.  351. — Circuits  for  Galvanometer  Constant. 

galvanometer,  S  the  shunt,  and  R  the  known  resistance. 
Usually  the  value  of  R  is  ^  of  a  megohm,  or  100,000  ohms. 
With  the  g-fg-  shunt  a  certain  deflection  will  be  obtained  when 
the  circuit  is  closed.  Obviously,  if  the  shunt  were  not  present 
the  deflection  would  be  1000  times  as  great,  because  only  -j^ 
of  the  current  passes  through  the  galvanometer.  Therefore  the 
total  deflection,  if  it  could  be  measured,  that  would  be  pro- 
duced through  the  galvanometer  and  the  100,000  ohms  resist- 
ance, would  be  the  deflection  noted  multiplied  by  IOOO.  If, 
now,  the  resistance,  R,  had  a  value  of  I  megohm  instead  of  TV 
megohm,  the  deflection  would  have  been  only  yV  as  great  as 
this.  Therefore  to  find  the  number  of  scale  divisions  deflec- 


442  AMERICAN    TELEPHONE   PRACJ^ICE. 

tions  which  the  galvanometer  alone  would  give  with  I  megohm 
in  circuit,  we  multiply  the  deflection  noted  by  1000  and  by  -^. 

In  general  we  may  say  :  to  find  the  working  constant,  multiply 
tJie  deflection  obtained  by  the  multiplying  power  of  the  shunt,  and 
by  the  value  of  the  known  resistance  in  megohms. 

As  a  numerical  example  let  us  assume  that  with  the  -y-J-j-  shunt 
and  the  -fa  megohm  resistance,  we  obtain  a  deflection  of  200 
scale  divisions,  then  the  working  constant  is 

200  x  1000  X  —  =  20,000. 
10 

In  other  words,  20,000  would  be  the  number  of  scale  divisions 
obtained  were  the  entire  current  from  the  battery  allowed  to 
pass  through  the  galvanometer  with  one  megohm  in  series. 

With  50  cells  of  battery  (45  or  50  volts),  the  constant  under 
ordinary  working  conditions  with  a  good  D'Arsonval  galva- 
nometer, will  be  from  10,000  to  25,000.  With  a  Thomson  instru- 
ment a  much  higher  constant  may  be  obtained.  Mr.  George 
D.  Hale  of  the  Western  Electric  Company's  cable-testing  de- 
partment uses  a  large  four-coil  Thomson  instrument  with  600 
volts  obtained  from  a  motor  generator.  With  this  he  obtains  a 
constant  of  528,000,  and  by  adjusting  the  suspension  for  greater 
delicacy  can  obtain  as  high  as  2,000,000.  Of  course  this  is 
entirely  impracticable  for  portable  instruments,  and  is,  in  fact, 
unnecessary,  as  good  work  may  be  done  with  a  constant  of 
2O,OOO.  In  ordinary  testing  a  battery  of  50  cells  is  sufficient. 
Of  course  a  higher  working  constant  may  be  obtained  with  a 
larger  battery,  and  frequently  100  cells  are  used. 


INSULATION   TESTS. 

One  of  the  principal  uses  of  the  galvanometer  in  line  testing 
is  in  the  measurement  of  insulation  resistance.  The  insulation 
resistance  of  any  line  or  conductor  is  the  joint  resistance  of  all 
the  leaks  from  the  line  to  the  ground  or  to  other  conductors. 
On  a  pole  line  every  insulator  forms  a  leak  to  earth,  and  on  a 
line  having  40  poles  to  the  mile  there  would  be  40  such  leaks  in 
parallel.  The  insulation  resistance  of  a  line  as  a  whole  varies 
inversely  as  its  length,  if  the  insulation  is  uniform.  Evidently, 
a  line  two  miles  long  would  have  one-half  as  great  an  insulation 
resistance  as  a  similar  line  one  mile  long,  because  on  the  latter 
there  would  be  only  half  as  many  leaks  as  on  the  former.  In 


TESTING.  443 

general  it  may  be  stated  that  a  line  n  miles  long  will  have  only 
—  as  great  an  insulation  resistance  as  a  similar  line  one  mile  in 

length.  In  order  to  obtain  a  standard  of  insulation  resistance 
independent  of  the  length  of  the  line,  it  is  convenient  to  express 
the  insulation  resistance  as  so(  many  megohms  per  mile.  The 
insulation  resistance  per  mile  is  found  by  multiplying  the  insula- 
tion of  the  line  as  a  whole  by  the  length  of  the  line  in  miles. 

In  order  to  measure  the  insulation  resistance  of  a  line  the 
constant  of  the  galvanometer  is  first  taken  and  then  the  known 
resistance  is  cut  out  of  circuit  and  the  line  insulation  resistance 
substituted  for  it.  Assuming  that  the  insulation  resistance  to 
be  measured  is  that  of  a  wire  in  a  cable,  the  terminals  of  the 
circuit  which  were  connected  with  resistance,^,  in  Fig.  351,  will 
be  connected  one  with  the  wire  and  the  other  with  the  sheath 
of  the  cable  as  shown  in  Fig.  352.  Care  must  be  taken  that  the 


Fig-  352. — Insulation  Resistance  of  Cable. 

wire  being  measured  is  carefully  insulated  from  the  sheath  at 
the  other  end  of  the  cable.  The  shunt,  S,  is  then  cut  out  of 
circuit  in  order  that  the  full  current  may  pass  through  the  gal- 
vanometer. Before  completing  the  circuit  with  the  cable  con- 
ductor and  sheath,  however,  the  key,  K,  should  be  closed  in 
parallel  with  the  galvanometer,  in  order  to  prevent  the  rush  of 
current  that  will  take  place  in  charging  the  cable,  from  causing 
the  needle  to  give  too  violent  a  kick.  After  a  short  time  the 
key  is  opened  and  all  of  the  current  diverted  through  the 
galvanometer.  The  galvanometer  then  receives  only  that  cur- 
rent which  leaks  from  the  core  of  the  cable  to  the  sheath 
through  the  insulation.  Under  these  circumstances  a  certain 
deflection  will  be  noted,  and  by  comparing  this  deflection  with 
the  constant  already  obtained  the  value  of  the  insulation  resist- 
ance in  megohms  is  readily  determined. 

To  illustrate,  suppose  that  a  deflection  of  75  scale  divisions  is 
obtained  with  the  apparatus  connected  as  in  Fig.  352.  If  the 
constant  is  20,000,  as  already  determined,  we  know  that  the 


444 


AMERICAN-    TELEPHONE   PRACTICE. 


insulation  resistance  must  be  20,000  divided  by  75,  or  266  meg- 
ohms, thus  indicating  that  the  total  insulation  resistance  of  the 
cable  is  266  megohms.  That  this  is  true  is  evident  from  the 
fact  that  the  constant,  20,000,  represents  the  number  of  scale 
divisions  deflection  that  would  be  obtained  were  only  one  meg- 
ohm in  the  circuit.  The  deflections  are  inversely  proportional 
to  the  resistance  in  the  circuit,  and  therefore  the  total  insulation 
resistance  is  equal  to  the  deflection  through  one  megohm 
divided  by  the  deflection  through  the  insulation  resistance,  or 
20,000  divided  by  75.  To  sum  up  these  operations  : 

1st.  Obtain  the  galvanometer  constant  or  deflection  obtained 
when  the  galvanometer  in  series  with  one  megohm  resistance  is 


Fig.  353. — Connections  for  Insulation  Test. 

subjected  to  the  potential  of  the  battery.  2d.  Find  the  deflec- 
tion obtained  when  the  galvanometer  and  insulation  resistance 
in  series  are  subjected  to  the  potential  of  the  battery.  3d. 
Divide  the  constant  by  the  deflection  obtained  through  the 
insulation  resistance,  the  result  being  the  insulation  resistance 
of  the  cable  expressed  in  megohms.  4th.  To  find  the  insulation 
resistance  per  mile,  multiply  the  total  insulation  resistance  by 
the  length  of  the  cable  in  miles. 

If  the  insulation  of  the  cable  is  low,  a  shunt  must  be  used  in 
obtaining  the  deflection  through  the  insulation  resistance.  If  the 
insulation  resistance  is  high,  the  deflection  will  be  small  and  no 
shunt  will  be  required.  The  purpose  of  the  shunt  is  merely  to 
keep  the  deflections  on  the  scale  so  that  they  may  be  read. 

In  Fig.  353  is  shown  a  convenient  arrangement  of  connections 
for  making  insulation  tests.  In  this,  B  is  the  battery  of  say 
50  cells,  R  the  -^  megohm  box,  5  the  shunt  box,  G  the  galva- 
nometer, and  V  a  convenient  switch  for  throwing  either  the  -^ 


TESTING.  445 

megohm  box  or  the  line  insulation  into  circuit  with  the  galva- 
nometer and  battery.  When  the  levers  of  the  switch,  F,  are  in 
the  position  represented  by  the  dotted  line,  the  circuits  are 
those  for  taking  constant  of  the  galvanometer,  and  when  in  the 
position  shown  by  full  lines,  the  circuits  are  those  for  obtaining 
the  deflection  through  the  insulation  of  the  cable.  Various 
forms  of  keys  for  changing  the  direction  of  the  battery  current 
through  the  galvanometer,  and  for  performing  other  switching 
operations  with  the  greatest  possible  convenience,  are  obtainable, 
and  form  an  important  part  of  all  testing  outfits.  The  scope  of 
this  work  will  not  permit  of  their  detailed  description. 

In  making  insulation  tests  the  resistance  of  the  lead  wires  to 
the  cable  or  line  need  not  be  taken  into  account.  It  is  a  matter 
of  the  greatest  importance,  however,  that  these  wires  are  perfectly 
insulated  from  each  other.  It  is  a  very  easy  matter  in  making 
tests  of  this  nature  to  measure  the  wrong  quantity. 

One  very  important  matter  in  connection  with  insulation  tests 
has  not  yet  been  spoken  of.  When  the  reading  is  being  taken, 
with  the  cable  or  line  insulation  in  circuit,  it  will  be  noticed  that 
a  maximum  deflection  is  obtained  at  first,  and  that  this  gradu- 
ally diminishes,  as  though  the  insulation  resistance  were  increas- 
ing. This  is  due  to  what  is  called  electrification,  a  phenomenon 
that  is  not  very  thoroughly  understood.  When  the  electromo- 
tive force  of  the  battery  is  first  applied  to  the  cable  or  line, 
there  is  a  sudden  rush  of  current,  due  to  the  charging  of  the 
conductors.  The  charges,  however,  apparently  soak  in  to  the 
insulation  to  a  slight  extent,  thus  allowing  more  current  to  flow 
to  the  conductors.  After  the  first  rush  due  to  the  first  charging 
of  the  conductors,  there  is  still  a  flow  of  current,  due  in  part  to 
this  soaking1  in,  and  in  part  to  the  actual  leakage  through  the 
insulation.  It  is  the  current .  due  to  the  latter  that  we  are 
concerned  with  in  insulation  measurements,  and  therefore  we 
must  wait  till  the  soaking  in  process  ceases,  when  the  flow  of 
current  will  be  practically  constant,  being  that  through  the 
insulation.  In  nearly  all  telephone-testing  work,  one  minute  is 
allowed  for  electrification,  after  which  the  reading  is  taken  of 
the  deflection.  When  one  is  thoroughly  familiar  with  his  instru- 
ments he  may  often,  where  great  accuracy  is  not  required, 
estimate  what  the  deflection  at  the  end  of  one  minute  will  be, 
by  watching  the  deflection  for  30  or  40  seconds.  This  method 
saves  time,  but  must  be  used  with  extreme  caution. 

With  a  constant  of  20,000  a  reading  taken  on  a  wire  in  a  piece 
of  good  new  telephone  cable,  one-quarter  mile  long,  would 


446  AMERICAN    TELEPHONE  PRACTICE, 

probably  show  a  deflection  of  8  or  10  scale  divisions  upon  the 
closure  of  the  key.  This  would  decrease  to  about  6  scale 
divisions  in  2  seconds,  and  to  about  2  scale  divisions  in  30  sec- 
onds, after  which  it  would  remain  constant.  The  reading  of  2 
divisions  at  the  end  of  the  minute  would  indicate  an  insulation 

r   20,000 
resistance  of =    IO,OOO  megohms,  or  2500  megohms  per 

mile. 

As  examples  of  deflections  on  the  different  wires  in  various 
cables  the  following  are  given  : 

Dry  paper  cable,  f  mile  long,  two  years  old.  Galvanometer 
constant  22,000:  Readings,  12-15-15-10-10-15-15-15-13  scale 
divisions. 

Another  dry  paper  cable,  2750  feet  long,  one  year  old.  Gal- 
vanometer constant  19000  :  Readings,  5-6-5-4-5-5-5-5-6,  etc. 

A  piece  of  jute  and  ozite  cable  five  years  old,  6000  feet  long, 
gave  the  following  with  a  constant  of  20,000:  7500-2500-1000- 
1000-600-800-900-800-1000.  It  was  necessary  to  use  the  TV 
shunt  in  taking  these  readings. 

Another  piece  of  the  same  kind  of  cable,  800  feet  long,  with 
a  constant  of  20,000,  gave  175-200-250-270-160-110-120-160- 
110-125. 

CAPACITY   TESTS. 

A  very  important  measurement,  especially  in  telephone  cables, 
is  the  determination  of  the  capacity  of  the  line  conductors  with 
respect  to  all  neighboring  conductors.  The  usual  method  of 
making  capacity  tests  is  to  note  the  deflection  produced  when  a 


B 
r>-  354-—  Capacity  Test. 


condenser  of  known  capacity,  after  having  been  charged  to  a 
known  potential,  is  discharged  suddenly  through  the  galvanom- 
eter, and  to  compare  this  with  the  deflection  obtained  when 
the  cable  or  conductor  being  measured,  after  being  charged  to 
the  same  potential,  is  discharged  through  the  galvanometer.  The 


TESTING.  447 

deflections  produced  under  these  circumstances  are  proportional 
to  the  charges,  and  therefore  to  the  capacities  of  the  standard 
condenser  and  the  line  or  cable.  The  circuits  for  obtaining  the 
deflection  produced  by  the  discharge  of  the  condenser  are 
shown  in  Fig.  354,  where  C  is  the  standard  condenser,  B  the 
battery,  and  G  the  galvanometer.  When  the  key  is  depressed 
the  condenser  is  charged  to  the  full  potential  of  the  battery,  B. 
The  key  is  then  suddenly  released,  thus  allowing  the  charge 
from  the  condenser  to  pass  through  the  galvanometer,  thus 
producing  a  certain  throw  of  the  needle.  The  connections  are 
then  made  as  shown  in  Fig.  355,  the  same  battery,  £,  being  used. 


IL 


will- 

R 

Fig-  355- — Capacity  Test. 

When  the  key  is  depressed  the  cable  is  charged,  and  when 
suddenly  released  this  charge  flows  through  the  galvanometer 
and  produces  another  throw  of  the  needle.  By  comparing  the 
throw  produced  by  the  charge  of  the  condenser  with  that  pro- 
duced by  the  charge  of  the  cable,  a  direct  comparison  may  be 
made  between  the  capacity  of  the  cable  and  that  of  the  con 
denser.  Thus,  if  with  the  -g-j-g-  shunt  the  discharge  from  the 
condenser  gave  a  deflection  of  100  scale  divisions,  the  capacity 
of  the  condenser  being  TV  microfarad,  and  if  with  the  same  shunt 
the  discharge  of  the  cable  produced  a  deflection  twice  as  great,  we 

would    know  that    the    capacity  of  the  cable  was  2  X  —  = 

microfarad. 

Convenient  connections  for  making  capacity  tests  are  shown 
in  Fig.  356,  where  G  is  the  galvanometer,  5  the  shunt,  C  the 
standard  condenser,  KK  discharge  keys,  V  the  selecting  switch, 
and,  B  a  battery  of  eight  or  ten  cells.  With  the  switch,  V,  at 
the  left  and  both  discharge  keys  depressed,  the  current  from  the 
battery  will  flow  into  the  condenser,  thus  charging  it.  Upon 
the  sudden  release  of  the  discharge  keys,  the  condenser  will  dis- 
charge through  the  galvanometer  and  shunt,  giving  a  deflection 
which  should  be  noted.  With  the  switch,  V,  at  the  right,  the 


448 


AMERICAN   TELEPHONE  PRACTICE. 


cable  may  be  charged  and  discharged  in  the  same  manner,  and 
the  deflection  produced  by  its  discharge  noted.  About  seven 
cells  of  battery  is  usually  sufficient  for  making  capacity  tests  on 
telephone  cables.  If  a  non-adjustable  condenser  only  is  avail- 
able, one  having  a  capacity  of  TV  microfarad  is  probably  most 
desirable.  For  accurate  work  a  subdivided  condenser,  having 
its  divisions  so  arranged  as  to  be  easily  connected  in  multiple  or 
in  series,  or  in  combinations  of  the  two,  is  very  desirable.  Then 


Fig-  356.— Circuits  for  Capacity  Test. 

the  condenser  capacity  may  be  varied  until  the  throw  from  the 
condenser  is  nearly  equal  to  that  from  the  cable,  thus  greatly 
minimizing  the  liability  to  error  in  the  results.  In  making 
capacity  tests  the  wire  under  test  should  be  carefully  insulated 
and  all  the  other  wires  in  the  cable  should  be  connected  together 
and  to  the  sheath  or  ground.  Fifteen  seconds  should  always  be 
allowed  for  the  charging  of  the  cable. 

If  d  is  the  throw  due  to  the  discharge  of  the  condenser,  d'  that 
to  the  discharge  of  the  cable,  K  \\\z  capacity  9f  the  condenser 
in  microfarads,  and  X  the  capacity  of  the  wire  being  measured, 
then 

X  :   K  ::  d1  :  d 

z-fx 

If  the  throws  of  the  galvanometer  are  too  large  to  be  measured, 
the  shunt  must  be  used.  In  this  case  dor  d'  in  the  formula  will 


TESTING.  449 

be  the  actual   throws  observed  multiplied  by  the  multiplying 
power  of  the  shunt. 


THE    LOCATION   OF   FAULTS. 

When  a  break  occurs  in  a  wire  in  a  line  or  cable,  the  ends 
remaining  insulated  from  other  wires  and  the  ground,  the  only 
recourse  is  to  capacity  tests.  The  capacity  of  the  two  parts  of 
the  wire  will  be  proportional  to  their  lengths,  the  wire  being 
uniform  in  size  and  in  its  relation  to  other  wires,  throughout  its 
length. 

We  may  locate  a  break  of  this  nature  in  several  ways. 

Measure  the  capacity  of  one  end  of  the  broken  wire,  then  go 
to  the  other  end  of  the  cable  and  do  the  same.  Calling  D  the 
length  of  the  cable  in  feet,  C  the  capacity  of  the  first  portion  of 
the  wire,  C  that  of  the  other,  and  X  the  distance  in  feet  to  the 
break  from  the  first  end,  then  : 

X  :  D  ::  C  :   C+  C 
CD 


When  a  good  wire  is  available,  and  this  is  usually  the  case, 
set  up  the  instruments  for  capacity  testing,  and  take  a  throw,  d, 
on  the  broken  wire,  another,  d',  on  the  good  wire,  and  a  third,  d" ', 
on  the  good  wire  with  the  broken  wire  connected  to  it  at  the 
far  end. 

Evidently  the  throw  on  the  whole  broken  wire  would  be 
d"  -  d'  +  d. 

Hence  where  D  and  X  have  the  same  significance  as  before 

X  :  d  ::  d  :  d"  -  d'  +  d 

r-          dD 

d"  -  d  +  d? 

The  location  of  breaks  is  much  complicated  by  the  presence 
of  poor  insulation  between  ruptured  portions,  and  between  other 
wires.  The  insulation  resistances  between  these  parts  should 
always  be  taken.  If  less  than  one  megohm,  the  results  obtained 
by  the  capacity  tests  should  not  be  relied  on,  and  other  methods 
too  complex  for  description  here  may  be  resorted  to.  It  seldom 
pays  to  open  a  lead-covered  telephone  cable  for  the  purpose  of 
joining  a  few  broken  wires,  the  expense  of  making  the  splice 
being  usually  in  excess  of  the  value  of  the  wires. 


45° 


AMERICAN   TELEPHONE   PRACTICE. 


The  location  of  crosses  or  grounds  is  rendered  somewhat 
difficult  by  the  fact  that  there  is  nearly  always  some  resistance 
in  the  fault  itself.  If  we  know  the  resistance  of  the  defective 
wire  and  have  no  good  wire  running  parallel  with  it,  we  may 
proceed  as  follows,  using  a  good  Wheatstone  bridge  : 

Measure  the  resistance  of  one  end  of  the  defective  wire 
through  the  fault  to  ground.  Do  the  same  at  the  other  end. 
Then  calling  R  the  total  resistance  of  the  wire  (either  known  or 
calculated  from  its  size  and  length),  R  the  measured  resistance 
from  the  first  end,  R"  that  from  the  other  end,  X  the  resistance 
from  the  first  end  to  the  fault,  Fthe  resistance  from  the  second 
end  to  the  fault,  and  Z  the  resistance  of  the  fault,  we  have : 

R  =  X  +  F. 
R1  =  X  +  Z. 
R"  =  Y  +  Z. 

Solving  these  for  X  and  F  we  have 

R  +  R  -  R" 


Y  = 


_  R  -  R  +  R" 
— j 

2 


which  values  are  independent  of  the  resistance  of  the  fault. 
Knowing  the  resistance  to  the  fault,  it  is  easy  to  compute  the 
distance  to  it,  from  the  resistance  per  foot  of  the  conductor. 

When  a  good  wire  is  available,  the  Varley  loop  test  should  be 
used,  as  it  is  more  accurate  than  the  method  just  described. 


FAULT 


ig-  357.— Varley  Loop  Test. 


For  this  a  Wheatstone  bridge  is  used,  and  connected  as  in  Fig. 
357.  The  good  and  bad  wires  are  joined  at  their  distant  ends, 
and  one  terminal  of  the  battery  connected  to  the  point,  e,  on 
the  bridge,  while  the  other  terminal  is  grounded.  It  is  not 


TESTING.  45  * 

difficult  to  see  that  the  partial  ground  or  fault  now  bears  the 
same  relation  to  the  bridge  as  the  point,  t,  in  the  diagram  of 
Fig.  342  ;  the  rheostat  arm  now  includes  the  resistance,  R,  plus 
the  resistance  of  the  bad  wire  to  the  fault,  while  the  unknown 
arm  includes  the  resistance  of  the  good  wire,  plus  the  resistance 
of  the  bad  wire  on  the  other  lside  of  the  fault. 

The  equation  of  the  bridge,  when  balanced,  then  becomes 

A       R  +  X 
B  ~~~-  C  +  Y' 

where  R  is  the  unplugged  resistance  of  the  rheostat,  X  the 
resistance  to  the  fault,  Y  the  resistance  beyond  the  fault,  and  C 
that  of  the  good  wire. 

Now  calling  L  the   resistance  of  the  loop  consisting  of  the 
good  and  bad  wires,  we  have 

L  =  X  +  Y+  C, 
or  C  +  Y=L-X. 

Substituting  this  in  the  second  member  of  the  equation  of  the 
bridge,  we  have 

A      R  +  x 

B~~     L-  X" 

A  L-  B  R 
whence^:        ^+^ 

which  is  independent  of  the  resistance  of  the  fault.  When  the 
two  ratio  arms  of  the  bridge  are  given  equal  values  we  have 
A  =  B,  and  the  equation  for  X  becomes  : 


Sometimes,  in  ordinary  paper  cables,  a  requirement  is  made 
that  a  rubber-covered  test  wire  shall  be  run  through  the  center 
of  the  cable,  so  that  at  least  one  good  wire  may  always  be  avail- 
able in  testing.  Where  no  good  wire  is  available,  a  separate  wire 
may  be  strung  to  be  used  as  the  return  in  this  test. 

If  the  lead  wires,  from  the  instruments  to  the  faulty  wire,  have 
appreciable  resistance,  this  should  be  measured,  and  deducted 
from  the  value  of  X.  After  this  the  distance  to  the  fault 
may  be  readily  obtained  from  the  resistance  per  foot  of  the 
conductor. 


INDEX. 


Abbott,  A.  V.,  199,  367 
Ader,  Clement,  23,  36 
Ahearn,  T.  F.,  46 
American  transfer  system,  334 
Ampere,  work  of,  i 
Anchor  for  guy,  379 

-  pole,  377 

Anders,  George  L.,  236,  312,  318 
Anders'  step-by-step  system,  312 
Arago  and  Davy,  work  of,  i 
Arrester  carbon,  273 

combined    carbon    and    heat 

coil,  278 

,  Hayes,  275 

,  McBerty,  277 

-,  Rolfe,  280 


Automatic  shunt,  86 

B 

Barrett,  Whittemore,  and  Craft,  330 

Battery  call  instruments,  circuits  of, 
101 

charging,  motor-generator,  116 

Batteries,  Fuller,  66 

,  gravity,  71 

,  Hayden,  64 

,  Le  Clanche,  63 

,  primary,  62 

,  storage,  73 

Bell,  Alexander  Graham,  6 

Bell's  early  instruments,  7 
magneto  telephone,  7 

Belt-driven  magneto,  114 

Berliner,  Emile,  n 

Birmingham  wire  gauge,  350 

Boiling  out  cable,  400 

Bonding  of  cable  sheaths,  422 

Bourse ul,  work  of,  5 

Braiding  on  cables,  393 

Branch    terminal     multiple     switch- 
board, 204 

Breaking  strength,  copper  wire,  358 

strength  of  wire,  347 

Breaks  in  line,  location  of,  449 

Bridging  bell  system,  Carty,  297 

telephone,  circuits  of,  99 

Brown  and  Sharpe  gauge,  350 

Buell,  Charles  E.,  243 

Busy  signal  on  party  lines,  306 

test  on  multiple  switch-boards, 

201,  203,  206,  260 


Cable,  boiling  out  of,  400 

—  construction,  underground,  409 

hangers,  396 

head,  Cook,  403 

head,  interior,  408 

head,  Moon,  401 

capacity  of,  391 

comparative  cost,  389 

drawing  in,  419 

dry-core,  392 

lead  covered,  sizes  of,  394 

overhead, 389 

rubber-covered,  389 

rubber-covered,  data  concern- 
ing, 39i 
saturated-core,  392 

—  sheaths  of,  393 

electrolysis  on,  421 

sizes  of  wire  in,  393 

—  splicing  of,  399 
stringing  of,  397 


—  terminals,  pot-head,  405 
testing  with  receiver,  425 


Calling  apparatus,  75 
apparatus  battery,  75 

apparatus,  commercial,  104 

Cant-hook,  371 
Capacity,  128 

of  telephone  lines,  133 

tests,  446 

Carbon  transmitter,  13 

transmitter,  action  of,  32 

Carry-hook,  369 

Carty,  J.  J.,   100,  136,   139,  241,  246, 

297 

bridging  bell  system,  297 

Carty's  experiments  in  line  induction, 

139 

Cement  arch  conduit,  416 
Charge,  128 

Circuits  of  telephone,  90 
Clamp  for  guy  wire,  381 

for  messenger  wire,  396 

Clay  conduits,  412 
Clearing-out  drop,  156 
Climbers,  Eastern,  384 

,  Western,  384 

Colvin,  F.  R.,  41 
Come-along,  383 
Common  battery  systems,  236 

,  Dean,  241,  249 

,  Hayes,  240,  254 


453 


454 


INDEX, 


Common  battery  systems,  house,  267 
multiple  board,  257 
repeating  coil,  240 
Scribner,  252 
series,  237 
Stone,  238 
storage    cell    at    subscriber's 


station,  242 

,  Stromberg-Carlson,  263 

Common  return  lines,  induction 


on, 


return  systems,  144 

return  wire,  position  of,  144 

return,  size  of,  146 


Concrete,  374 

—  for  conduit,  415 
Condenser,  capacity  of,  129 
Conduit,  cement-arch,  416 

— ,  clay,  412 

,  concrete  for,  415 

,  creosoted  wood,  411 

— ,  Johnston  distributing,  413 

,  mandrel  for,  414 

,  mortar  for,  416 

— ,  multiple  duct,  412 

,  obstructions  in  laying,  418 

,  open  box,  410 

— ,  pump  log,  410 

,  requirements  of,  409 

,  single  du9t,  clay,  414 

— ,  trench  for,  414 

,  vitrified  clay,  412 

Constant  of  galvanometer,  441 

Construction  tools,  370 

Continuity  tests,  427 

Cook,  F.  B.,  88,  165 

Cook-Beach  transfer  system,  232 

Copper  wire,  355 

wire  breaking  strength  of,  358 

—  wire  data  concerning,  356 

wire  specifications,  357 

Creosoted  wood  conduit,  411 

Cresoting  of  poles,  364 

Cross-arms,  attachment  to  poles,  365 

,  sizes  of,  364 

,  spacing  of  holes,  364 

Cross- talk,  139 

D 

D'Arsonval  galvanometer,  435 
Dead-man,  372 
Dean  thermopile  system,  245 
Dean,  W.  W.,  241,  243,  244,  249,  327 
Desk  set,  101 

,  circuits  of,  102 

Diaphragms,  receiver,  29 
Dickerson,  E.  N.,  308 
Dielectric,  129 
Digging  bar,  370 
Distributing  boards,  281 

,  Ford  &  Lenfest,  286 

— ,  Hibbard,  283 
Disturbances  in  telephone  lines,  136 


Drawing  in  of  cables,  419 

Drop,  American,  162 

and  Jack  combined,  American, 

179 
and  Jack  combined,  Western, 

177 

,  Keystone,  162 

,  Warner,  160 

,  electrically  self -restoring,  173 

,  mechanically     self-restoring. 

176 

Dry-core  cables,  392 
Du  Moncel,  n 
Dynamometer,  382 

E 

Edison,  Thomas  A.,  13,  15,  119 
Edison's  transmitter,  13 
Electrification,  445 
Electrolysis.  421 
Electromagnetic  induction,  125 
induction  disturbances  due  to, 

137 

Electrom  agnetism,  2 
Electromotive  force,  active,  131 
Electrolytic  cell,  244 
Electropoin  fluid,  67 
Electrostatic  induction,  129 
induction  disturbances  due  to, 

138 
Erdman,  A.  W.,  121 

,  Relay,  121 

Ericsson,  L.  M.,  42 
Explosions  in  manholes,  421 
Express  system,  217 


Faraday,  2 

Faults,  location  of,  449 

Fessenden,  Professor  R.  A.,  33 

Field  of  force,  125 

Ford  and  Lenfest,  286 

distributing  board,  286 

Fuller  cell,  Standard,  66 


Gaining  template,  369 
Gains  in  poles,  369 
Galvanized  steel  strands,  395 
Galvanizing  of  iron  wire,  352 
Galvanometer,  435 

constant  of,  441 

shunt,  439 

Gas  in  manholes,  421 
Gauge,  circular  wire,  349 

,  micrometer,  349 

Generator,  Holtzer-Cabot,  105 
,  Western  Telephone  Construc- 
tion Co.,  104 

,  Williams,  108 

,  Williams- Abbott,  107 

,  constantly  driven,  113 


INDEX. 


455 


Gharky,  W.  D.,  47 

Gravity  cell,  71 

Gray,  Elisha,  6,  339 

Gray's  transmitter,  10 

Ground  return  systems,  144 

Grounded  to  metallic  circuits,  connec- 
tion of,  149 

Grounds  or  crosses,  location  of,  450 

Guy,  anchor,  379  < 
clamp,  381 

Guying,  head,  375 

,  side,  375 

Guy,  Y-,  378 

H 

Hampton,  217,  320-21 

Hand  barrow,  382 

Hard  rubber,  imitation,  22 

Harmonic  signaling,  338 

Hayes,  H.  V.,  240,  255,  275 

Head  guys,  375 

Henry,  Joseph,  i 

Hibbard,  Angus  S.,  281,  322 

distributing  board,  283 

History  of  the  telephone,  i 
Holtzer-Cabot  house  system,  269 
Hook-switch,  90 

,  Warner,  94 

House,  Royal  E.,  8 
House  systems,  265 
Hughes'  microphone,  14 

,  David  E.,  13 

Hunning,  Henry,  15 

I 

Impedance,  127,  132 
Incandescent  lamps  as  signals,  192 
Induction,  electromagnetic,  125 
Induction  coil,  action  of,  16,  53 
coil  in  base  of  transmitter  arm 

60 

coil,  commercial  types,  58 

coils,  comparison  of,  5 

coils,  selection  of,  57 

coils,  Varley,  59 

Iron  wire,  352 

wire,  data  concerning,  355 

wire,  grades  of,  353 

wire,  specifications  for,  354 

Imitation  hard  rubber,  22 
Insulation   resistance,   measurement 

of,  442 
Insulators,  tests  of,  367 

J 

Jack-strip,  210 
Jacques,  W.  W.,  50 
Johnston,  H.  W.,  413 
Joint,  Lillie,  386 

,  Mclntire,  386 

,  Western  Union,  386 

Jumper  wires,  293 


K 


Kellogg,  Milo  G.,  213 

divided  multiple  switchboard, 

214 
Knudsen,  A.  M.,  178 


Lag  screws,  365 

Lamp  signal  controlled  by  relay,  195 
—  directly  in  line  circuit,  193 

,  life  of,  199 

switch-boards,  192 

Leakage,  disturbances  due  to,  137 
Le  Clanche  batteries,  63 
Lenfest  and  Ford,  286 
Lightipe,  J.  A.,  342 
Lightning  arrester,  American,  273 

arrester,  ordinary  form,  272 

arrester,  Western,  274 

Lillie  wire  joint,  386 

Line  disturbances,  remedy  for,  140 

Lines  of  force,  3 

Listening  and  ringing  apparatus,  163; 

and  ringing    key,  American, 

167 

and  ringing  key.  Cook.  165 

and    ringing     key,     Western. 

Telephone  Construction  Co.,  169 

key,  American,  169 

key  plug  socket,  172 

Location  of  breaks  in  line,  449 

of  crosses  in  grounds,  450* 

of  faults,  449 

Lockout  systems   on  party  lines,  301 
Lockwood,  Thomas  D.,  312 
Loop  test,  Varley,  450 
Low,  George  P.,  227 

M 

Magneto  generator,  75 

generator,  voltage  of,  83 

testing  set,  424 

testing  set,  errors  from  use  of, 

425 

Magnets,  permanent,  81 
Make-and-break  telephone,  5 
Manholes,  419 

explosions,  421 

McBerty,  F.  R.,  243,  277,  325 
McCluer  system,  146 
McDonnell,  J.  L.,  315 
Mclntire  sleeve  joint,  386 
Measurement  of  capacity,  446 

of  insulation  resistance,  442 

of  resistance  with  Wheatstona 

bridge,  431 

Meissner  ringing  device,  171 
Messenger  wire,  394 

wire  clamp,  396 

Micrometer  gauge,  349 
Mile-ohm,  348 


456 


INDEX. 


Morse  telegraph,  4 
Mortar  for  conduit,  416 
Motor-generator,  115 

for  battery  charging,  116 

Multiple  duct  conduits,  412 

switchboard,  200 

—  switchboard,  branch  terminal, 
204 

switchboard,  series,  201 

transmitter  circuits,  246 

N 
Ness  automatic  switch,  269 

O 

O'Connell,  J.  J.,  163,  192 

O'Connell  key,  163 

Oersted,  work  of,  i 

Ohm's  law,  124 

Overhead  cable  construction,  389 


Packing,  45 

Party    line,  Anders'    strength    and 
polarity,  318 

,     Barrett,     Whittemore,     and 

Craft,  330 

,  bridged  grounded,  297 

,  busy  signal  on,  306 

,  classification  of,  294 

-,  Currier  and  Rice,  339 

,  Dean,  327 

,  Dickerson,  308 

— ,  Gray  and  Pope,  339 

,  harmonic  selective  signaling, 

338 

-,  Harter,  344 

,  Hibbard,  322 

— ,  Lightipe,  342 

,  Lockwood,  312 

,  McBerty,  325 

,  non-selective,  294 

— ,  Reid  and  McDonnell,  313 

,  Sabin  and  Hampton,  320 

,  secrecy  on,  301 

,  series  grounded,  295 

— ,  step-by-step  selective  signal- 
ing, 308 

— ,  strength  and  polarity,  318 

,  Wood,  F.  B.,  315 

Payne,  George  F.,  47 

Permanent  magnets,  81 

Permeability,  80 

Phelps,  George  M.,  13 

Phonograph,  busy  test,  227 

Pike-poles,  371 

Pilot  lamp,  198 

Plug    for    branch    terminal    system, 

212 

—  for  metallic  circuit  system,  158 
listening  device,  170 


Polarized  bell,  78 

,  construction  of,  84 

,  Holtzer-Cabot,  106 

,  Williams,  in 

,  Williams- Abbott,  108 

Pole,  anchor,  377 

brace,  381 

buck,  369 

guards,  362 

holes,  depths  of,  371 

line  construction,  360 

line,  route  of,  368 

top  terminal,  403 

tops,  grading  of,  368 

Poles,  creosoting  of,  364 
heights  of,  362 

life  of,  360 

number  to  the  carload,  363 

numbers  to  the  mile,  361 

sizes  of,  360 

vulcanizing  of,  364 

weight  of,  363 

woods  for,  360 

Pope,  Frank  L.,  339 
Pot-head  terminals,  405 

Power    circuit  and    telephone    line, 

388 

Power  generators,  113 
Preece  and  Stubbs,  55 
Protection  of  cable  sheaths  from 

electrolysis,  44 
Protective  devices,  272 
Pump-log  conduits,  410 

R 

Raising  poles,  372 

Receiver,  Ader,  23 

,  American  Electric  Telephone 

Co.,  27 

,  Bell  double-pole,  25 

,  Bell  single-pole,  19 

cords,  30 

diaphragm,  29 

,  efficiency  of,  18 

,  Ericsson,  28 

,  faults  in,  22 

,  Holtzer-Cabot  Electric  Co.,  27 

,  Stromberg-Carlson,  25 

,  Western  Telephone  Construc- 
tion Co.,  24 

Reid,  R.  T.,  315 

Reis,  Philip,  5 

telephone  instruments,  5 

Relay  circuits,  simple,  118 

circuits,  two-way,  113 

,  Erdman,  121 

lamp  system,  195 

,  Stone,  122 

telephone,  118 

Repeater  telephone,  118 

Repeating  coils,  149 

Resistance  measurement  with 
Wheatstone  bridge,  431 


INDEX. 


457 


Ringer,  78 
Rodding,  419 
Rolfe,  C.  A.,  280 
Rough  tests,  424 
Route  of  pole  line,  368 


Sabin,  John  I.,  217,  315,  320 

Saturated  core  cables,  392 

Scribner,   Charles   E.,   195,   198,  243, 

252,  257,  302 

Secrecy  on  party  lines,  301 
Self-induction,   124 

,  effect  of,  126 

Self-restoring  switch-board  drops,  1 73 
Series,  multiple  switch-board,  201 
telephone,  circuits  of,  96 

transmitter  circuit,  248 

Sewall  cement-arch  conduit,  416 

Shovels,  370 

Shunt,  automatic,  86 

,  centrifugal,  87 

,  Cook,  88 

for  galvanometers,  439 

,  Holtzer-Cabot,  88 

,  Western  Electric,  86 

— ,  Western  Telephone  Construc- 
tion Co.,  87 

— ,  Williams,  89 
Side  guys,  375 
Sound  waves,  6 
Specifications  for  copper  wire,  357 

—  for  iron  wire,  354 
Specific  inductive  capacity,  129 
Splicing  of  cable,  399 

Spring  jack,  153 

— ,  American,  158 
,  Keystone,  158 

— ,  metallic  circuit,  157 

—  for  multiple  switch-boards,  210 
Steam  power  for  drawing  in  cables, 

420 

Steel  strands,  395 

Step-by-step  mechanisms,  308 

Sterling  Company,   cable   terminals, 
408 

Electric    Company's     switch- 
board, 187 

St.  Louis  Bell  exchange,  260 

Stone,  John  S.,  122,  238,  243 
relay,  122 

Storage  batteries,  care  of,  74 

Strains  on  pole  lines,  375 

Strength   and  polarity   signaling  on 
party  lines,  318 

Stringing  of  wires,  382 

Sturgeon,  William,  i 

Supporting  strand  for  aerial  cable, 

^  395 

Sutton  transmitter,  42 

Switch-board  drop,  154 

for  grounded  lines,  circuits  of, 


Switch-board,  metallic  circuit,  159 
for  small  exchanges,  153,  183 


Tamping  bar,  372 
Telephone  lines,  136 
Telescope  for  galvanometer,  438 
Tensile  strength  of  wire,  347 
Tension  in  wires,  383 
Terminal  pole,  376 
— t  pole  top,  403 
Testing,  424 

—  set,  magneto,  424 
Tests  for  capacity,  446 

—  for  continuity,  427 

—  for  crosses  and  grounds,  425 

—  for  electrolysis,  421 

—  with  receiver,  425 
Thermal  arrester,  Hayes,  275 

,  McBerty,  277 

Thermopile  system,  Dean,  245 
Thomson  galvanometer,  435 
Tie,  Kelvin,  384 

— ,  latest  method,  385 

— ,  ordinary,  384 
Tin  in  cable  sheaths,  393 
Transfer  systems,  216 

— ,  Western  Telephone  Construc- 
tion Co.,  227 
Transpositions,  143 

— ,  method  of  making,  387 
Trench  for  conduits,  414 
Transmitter,  Ader,  36 

,  Ahearn,  46 

,  Berliner's,  n 

,  Universal,  37 

,  Blake,  34 

,  Carbon,  13 

,  Clamond,  37 

,  Colvin,  41 

,  Crossley,  35 

— ,  D'Arsonval,  36 

,  Ericsson,  42 

,  Gower,  36 

,  Runnings',  15 

— ,  Jacques,  50 

,  Johnson,  36 

,  multiple-electrode,  36 

,  Payne  and  Gharky,  47 

,  "  solid  back,"  38 

— ,  Sutton,  41 

,  Turnbull,  36 

,  variable  resistance,  10 

,  Western  Telephone  Construc- 
tion Co.,  43 
White, 


,    38 

Tubular  drops,  160 


U 


Underground      cable     construction, 
409 


INDEX. 


Variable  resistance  transmitter,  10 
Varley  coils,  112 

—  loop  test,  450 
Viele,  F.  S.,  398 
Vitrified  clay  conduit,  412 
Vulcanizing  of  poles,  364 


W 


Warner,  94,  160 

—  drop,  1 60 

—  hook-switch,  94 
Waves  of  sound,  6 
Weight  per  mile-ohm,  348 
Western  switch-board,  183 

-  Union  wire  joint,  386 
Wheatstone  bridge,  428 
White,  A.  C.,  38,  237 
Williams,  J.  A.,  89,  108,  in 
Wilmington,  Del.,  switch-board,  232      Y-Guy,  378 


Wire,  breaking  strength  of,  347 

,  conductivity  of,  347 

— ,  copper,  355 

,  copper,  specifications,  357 

for  guying,  381 

for  telephone  use,  347 

gauges,  348 

gauge,  Birmingham,  350 

\  SauSe   Brown    and    Sharpe. 

350 

gauges,  table  of,  351 

iron,  352 

iron,  grades  of,  353 

iron,  specifications  for,  354 

resistance  of,  348 

tensile  strength  of,  347 

tension  in,  38 -• 

Wood,  F.  B.,  315 


THE   END. 


YC   19357 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


